Blacks
The "ominous" black represents the void of space ("vacuum"). It also signifies "death" (not a stretch, for obvious reasons) and it, also, means "hazard". These are (can be) contextual but those interchangeable meanings are pretty much closely related.
The (all) 5 "black" signs/symbols are a very interesting bunch, for various reasons.
[5.] Airlock
This one is a pretty interesting one. Contrasting with the, until now, continuous, unbroken, safe, outer (red) layer, there are several discontinuities, suggesting that such livable (pressurized) condition isn’t permanent. As there is no misalignment (or even a big gap) it seems that both states (pressurized/unpressurized, safe/unsafe) are reversible or can be alternated. This seem to imply (closed, but openable) doors. This notion seems reinforced, firstly, by the placement (if one carries here the significance of “floor”-“ceiling” seen in, for instance, sign/symbol #2) in the "walls" but also the relative size (smaller than the other 2 red border segments).
So. Until now. There are 2 "doors" in the "walls" and the space in between is equally filled/divided in black (vacuum; death; hazard) and white (environment where a human can live/survive "normally"). All of the previous already fully (and capably) defines "Airlock" (as intended). But I contend that is another fine detail in "how" both areas (black and white) are distributed. And by this I mean the shape -- triangle(s) -- and (assuming triangle/triangles) the orientation (as there are 4 possible configurations using 2 complementary right angled triangles). What I surmise is that the chosen configuration (and the equilateral complementary triangles) is no coincidence but has an underlying logic governing it.
This "conviction" rests (or depends) on one assumption. Established that the sign/symbol represents an "Airlock" then both ("wall") doors connect to the very different environments to which the "Airlock" constitutes a buffer/interface: one connects to the spacecraft itself (i.e., the pressurized zone) or better yet, "Inside", and the other gives access to outer space -- or more correctly -- to the spacecraft exterior (i.e., "outside"). With that in mind a question emerges: “which one is which?. There are two ways to go about resolving this doubt: guess (or estimate), if no context is accessible, or, see the context (in this case the other signs/symbols). As it turns out the next (#6) sign/symbol denotes (indirectly) that left is "inside" and right is "outside" (which seems to validate that top-down, left-right is the convention used throughout the standard). So, that is (are) the assumption(s): the "left door" accesses the "inside" and the "right door" accesses the "outside". With that notion in place, one can now look to what is represented between the "doors". The "division" in half means that half the time that place is pressurized and the other half is not. It follows then, that immediately adjacent to the "left door" should be an all-white ("pressurized") representation and, symmetrically, immediately adjacent to the "right door" should be an all-black ("vacuum") representation (discarding/discounting the visual white separation, which is there and in many other signs/symbols, for the sake of clarity). Extending the left white and the right black in each other directions would result in 2 blocks (rectangles) of white (left) and black (right). But. Time have to be factored in the design. Because airlocks are used to equalize pressure. This means that they go to pressurized to depressurized (decrease pressure, pressure goes down) and vice versa (increase pressure, i.e., goes up). So it makes every bit of sense to represent that descent/ascent with a slope/slant/angle (meaning, triangles). And. From left-to-right (white-to-black) descending (pressure going down) and from right-to-left (black-to-white) ascending (pressure going up). Which is exactly how it is designed. QED.
BTW, this logic would be flawed if down-up, right-left convention was the one in place here. That goes without saying, OC.
[6.] Bulkhead Door
Another interesting one. Drawing from the previous sign/symbol, there is a "door" in the middle dividing the pressurized (left) and unpressurized (right) areas. The most interesting part of this design is that the protective layer (the red border) doesn't stop at the "supporting wall" (where the "door" is). It is prolonged. This seems to heavily imply that the "door" is an interior one (not like the right one in the previous sign/symbol). An interior door is called a bulkhead door so the representation seems about right.
Plus. This is clearly also a warning sign. In a spacecraft, a closed (or locked) bulkhead door does not ensure the same level of pressurization on the other side (of that same door) and that is the somber message that this clever design is trying to impart.
[7.] Non-Pressurized Area Beyond
Several things to mention about this one. Firstly, contrasting with the previous one ,there is definitely no pressurization there (so, the door clearly gives access to a non-pressurized area). Secondly, that area seems to be inside the spacecraft (albeit the gap at the top, which seems to indicate a possibility of some part to be outside, or, even, an opening, to exist). Lastly, the "odd" thing: the orientation seems “off” (the "door" is at the bottom, until now, the "floor"/"ground"). The motivation for this, to my mind, seems to be the visual resemblance with sign/symbol #6, if the "usual" (until now) orientation were used, the two signs/symbols would, sort-of, “clash” and would not be so visually meaningful (one can view this by rotating the sign/symbol 90º ,clockwise). The changing direction seems a small price to pay to achieve a greater clarity and distinctiveness (at the small price of breaking the "normal" here).
[13.] Artificial Gravity Area, Non-Pressurized, Suit Required
This is the direct counterpart of sign/symbol #2, but for a non-pressurized area. So. The (alive) human figure is still there but fully enclosed in a pressurized suit (the "head" representing portion is clearly depicted, to confirm this). The "floor" is still there to “ground” the suit-wearing human. The quantity of "ground" (red block) seems to indicate an exterior (to the spacecraft) but there is (enough) ambiguity there (I think) to be able to signify also some internal part of the spacecraft.
The main difference between this (#13) sign/symbol (and, incidentally, the next one, #14) with the previous one (#7) is that these 2 add information about gravity (which isn't at all informed in #7). The commonality between these 3 (“blacks”, until now) is that they definitely signalize that there is no pressure/pressurization present at the signalized areas.
[14.] No Pressure/Gravity, Suit Required
This is the direct counterpart of sign/symbol #3, but for a non-pressurized area. That added to what was said for the previous sign/symbol warrant no more commentary (as it would be, mainly, redundant).
