Here is my (slightly) eduacated guess.
Blood carries your nutrients. Less blood equals less nutrients to your muscles. Negative effect.
When the body is low on fluids, it spends it's energy to replace them, possibly pulling resources away from muscle growth.
I could be way off. Since plasma is mostly fluid, not red or white blood cells, it may have no effect as long as you stay hydrated.
I personally have donated blood and I feel shitty for the day, don't know about plasma.
Back in the school days, I knew lot's of guys who would sell plasma for beer money. Yeah, that result in negative effects.
Good question, hopefully somebody has a more solid answer than me.