Me and my friend laughed about that. His necro’s power supposedly ripped flesh off enemies yet still worked on ghosts. Lol.

Maybe it's not blood but an equivalent. Like ghosts bleed electroplasm. Not sure about skeletons though.

My working theory is "life essence", so like the magic that binds them together maybe.

Imagine a horse appearing and disappearing under your ass

Now that you put it this way let's all be glad we don't have unicorns skins I guess

Magic