Blending them into Life and Darkness would work, but that'd seriously bloat those two decks. Which, of course, means you'd have to make similar new cards for all the other decks. Which is a lot of work! If someone's up to it, that's great.
Although, I do think the idea of Parasite works better with Death than Darkness. I know in application the abilities fit better with Darkness (Darkness already has some parasitic abilities), but the theme works better with Death.