Maybe I'm reading the question wrong...  But I would just create four collision lines, one for top, bottom, left, and right for each block.  Then if the ball hits a block's left collision line, it will act in the way you want it to, the same for the block's right collision line..

Edit:  Just to explain how you would do this encase you ask.  You would make four "collision boxes", one for left, right, top, bottom in your block class.  This is very easy to setup based on your blocks X and Y values.  Then you would just make a line pretty much that outlines the block for collision checking.  You can even have the ball hit the corner of the block - meaning it hit the left and bottom lines then move the in proper direction.

Maybe I'm reading the question wrong...  But I would just create four collision lines, one for top, bottom, left, and right for each block.  Then if the ball hits a block's left collision line, it will act in the way you want it to, the same for the block's right collision line..