I want to know why the men in these games are blue. I also noticed that in another game, where the uncle was again "transparent" and blue.
From a game design perspective, the existence of these characters has more meaning for their presence than their looks (these brows, though, are something else).
For example,
Custom Maid 3D series do this for MC and Guests: you need a visual indication to tell where your character and the guests are located, but the way they look is irrelevant
Another example for a different use case:
Beelzebub's HomeGuard, HomeGuard 2, Convenience Store Z,
Hypnos f*ck, etc. They have a specific strategy:
1. Fully develop a base game with a complete story involving many characters. The main characters are fully detailed, then there are also characters who don't give much to the story, beside the fact that they have to be there, displayed as shadow figures.
2. Eventually, these characters are involved in future DLCs where they eventually get a complete appearance, plus additional story lines branching and merging with the main story.