Help with my Magic System

Try to think of limits in your magic system. Your light magic already has some in place (only fairyblood allows its use, and it is as powerful as the amount of fairyblood one has), but those are very general. In this case, because you’re dealing with fairies, I’d suggest looking at real world ‘magic’, for example the belief that iron could hurt fairies. Does the dark magic have any limits yet?

 

I think you have an excellent base for your system. With what you've covered so far, I'd recommend intentionally adding parallels between the dark and light magic. If light magic is countered by something, have darkness be countered by something connected but somewhat opposite.
This stage of adding definition to your magic is a tricky one. At the risk of sounding too self-promotional, I've got a number of posts on this topic over on my blog. I recommend starting with this one. Stage 3 of Building Marvelous Magic: Defining Your Magic ⋆ C. R. Rowenson
Don't hesitate to ask if anything doesn't make sense.