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

Related tags

blue_mouth, blue_clitoris, blue_inner_pussy, blue_anus, blue_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 blue_tongue without seeing that the rest of the mouth (or any other internal surface) is blue is not enough for blue_insides.
Same for just seeing a blue_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.