Grass blocks have random black parts below the overlay that appear when viewed from far away.
This is probably due to the change to OpenGL 3.2, and trying to optimize grass blocks. Grass blocks have a special property which allows them to have an overlay like it does, without inherently supporting transparency. Something must have broken with that property, which causes parts to appear black.

(Note: This does not only happen in swamps, that was just where I was when I took the screenshots)