You could size the black border to square up the blocks. If the blocks are 6-1/2 x 7-1/2, which is what it looks like to me, you could 'square' them all to that size, and then put 1" black border on 2 sides, and 1/2" black border on the other 2, to make squares that are 8-1/2 unfinished. Then you are back on plan.
Also, if all the blocks are like the one in the pic, it looks like you could square them to 7x7-1/2 and your borders could be 3/4 and 1/2", and the variance would be less obvious