Well, depends on just how big the blocks are. if you take two finished blocks and lay them on top of the black and slide them closer and farther apart you will see what works best for your eye. Trust your eye and remember to stand away or look in a mirror over your shoulder and you get a different perspective on the subject. My guess is 1" if doing it around each block and 1 1/2" if you try around four blocks together. Hope this helps and it is going to be fabulous whatever you choose!
peace :D