They look great both ways, but I'm leaning to the blue. But the yellow looks so fresh! I guess it depends on the recipient and their colors, as well as the colors of the room in which it would be used.
Have you thought about a 1/8" border or flange in a darker blue around each block to really frame them? Perhaps lighten the blue color just a little, it is kinda dull in color.
My DH just weighed in, he likes the yellow, said it frames the blocks, and that particular blue looks dull. But it could just be the pic