Are you blending the same color with itself only at different opacities? (I assume so because installing Sai on a Mac requires a lot of annoying workarounds.) It can only become more solid if you don't pick a different color.
BTW, I'm not sure what your saving problem is. Are you flatting your image?
Mm, I don't know, it's probably that. It only did this with some colours. I'll try it.
Well, of course no if I need the layers. SAI bugs sometimes on my laptop and I can't do anything about it. So when I wanted to save my drawing it went like a mosaic thing. Fortunately I saved it on png to try a few things on GIMP, so I was able to continue. But all the layers merged. u_u