Well I definitely don't think we should get rid of the hair and eye color tags altogether. I mean, tags are used to search for things. How are you going to look up characters with, say, red hair AND green eyes (or any permutation of hair and eye color) if we don't use them anymore? Sometimes it's hard to remember the exact name of an artist or character of a post you're trying to find.

As for the black eyes thing, I do use it when the characters eyes are not, in fact, totally black. If a character has black hair and their eyes exactly match the color(s) used for their hair, I will tag the eyes as being black (instead of gray).
If you can discern black pupils separate from the iris, then the iris has a color which is not black. There are very few posts with truly fully black eyes. If the color is indiscernible, whether because of the view angle or the lighting, it is better to not tag the eye color. To tag anything but truly black eyes as black_eyes instead of tagging grey_eyes for grey irides, we should just alias grey_eyes to black_eyes.
I agree that there is a subjective aspect to these tags... Even aside from the methods people use for examining the structure of depicted eyes/hair to determine its color, there are differences in how individuals see color, differences in how color is displayed by computer screens (my desktop, laptop, and phone all display color differently from each other), and differences in what ranges and shades of colors people personally associate with various color terms.

As an example, for post #237596 that was just uploaded, viewing it from my desktop computer I would classify the character on the (viewer's) right as having orange eyes and the character on the left as having either yellow or orange, perhaps closer to yellow. But on my phone's screen, which has more vibrant/vivid coloration (contrast or saturation perhaps?), both sets of eyes appear to be orange to me.

Kiho said:
If the color is indiscernible, whether because of the view angle or the lighting, it is better to not tag the eye color.
I would agree with this in cases of absolute failure to determine the color, but if someone manages to determine that (in their perspective) the eyes/hair are either of two potential colors, I would advocate for them tagging at least one of those colors if not both. That way the people whose perspectives err on the side of the tags that are applied can still benefit from them.

As for removing the tags entirely, I am unsure if the merit in this is worth what would be lost. There is the issue of reducing subjectivity in tagging and thereby upholding Konachan's culture and its quality standards, as well as the reduction of tag clutter (although I've personally never minded "tag clutter"). But it's not as if these tags are useless, is it? Personally I think it wouldn't be such a problem for Konachan to establish an understanding that the color tags are never going to be applied perfectly and to make an exception to the anti-subjectivity policies just for them.
On another note, is the wiki page for ass accurate? It says to only tag if the buttocks in question are the most prominent feature of the image. So then in post #237592 , where there is a prominent ass but it isn't the most prominent feature, it would be left untagged? Is that how people have been using the tag, and want to be using it?
Besides the obvious reason (presenting), I also tag ass if it is one of the focal points of the image, not necessarily the most prominent feature. I will also tag it if the primary character(s) of interest are facing backwards (not front facing) to indicate the viewing perspective similar to Not tagging breasts if a nude or topless character is visible only from the back. I don't tag it for any characters in the background. I generally don't tag for background characters at all.