It is when we see that internal surfaces of a body are black (instead of red).
It can include seeing black : interior of mouth, internal organs, pussy, anus, inside of nose, and other inside surfaces of body.
No requirement for internal view.

Related tags

black_mouth, black_pussy, black_anus, black_cloaca, black_internal.

Inside surfaces colors

• black_insides
• purple_insides
• blue_insides
• green_insides
• yellow_insides
• orange_insides
• red_insides
• pink_insides
• white_insides

Do not Extrapolate

Seeing just a black_tongue without seeing that the rest of the mouth (or any other internal surface) is black is not enough for black_insides.
Same for just seeing a black_body and not more.
Also when the transition between outside and inside is not the same color than the inside color, the <color>_insides tag should be from the color of inside and not from the transition.