I know what you're talking about. I would call it a "half quarter square triangle", but it must have a proper name.
If you put four of the blocks together with the lighter of the small triangles toward the outside, you get "Double Pinwheel" according to "Around the Block" by Judy Hopkins, or "Windmill" according to Better Homes and Gardens' "100 Quilt Blocks".
I need to do more research...