I'm not sure that a betta is the kind of fish that particularly cares about their gravel color. However, many smaller, schooling fish (and even bigger ones) especially can be skittish if they are placed in a tank with very light gravel. Naturally, most fish come from areas where the bottom is dark brown to black. It helps the fish feel secure because they "know" that it makes them much less visible from the surface, ie, their silouette is much harder to make out. With a lighter gravel, the fish would be just fine, but they may feel a little scared/skittish and therefore their colors may not be as vibrant as possible.
Lighter colors can also often attract the eye's attention - away from the fish. If your blue gravel is brighter than the fish, imo, it takes away from the fish itself. Not only can lighter colors be distracting, but they can wash out the fish's color. Darker colors tend to highlight the fish (much like a black background forces you to concentrate on the fish, while a flourescent pink one may draw your attention instead) and can make them feel more secure, therefore giving them (or at least an illusion to) brighter colors.
I have a lightish fine grained gravel/sand in my 29g. In that tank, my boesmani is light in color. It had to get moved to another tank temporarily...to one that had black gravel/black background. The boesmani was much darker in this tank, having dark blue and orange rather than light yellow/green.
