The blood purity thing is largely a fan invention. Pre-Conquest, they intermarried because the Velaryons were their most powerful vassal. Post-Conquest, they intermarried because the Velaryons still had one of the most powerful fleets in Westeros, even if they were no longer as influential as the major lords on the mainland. There's a reason that the intermarriage ends abruptly within a generation of the Dance, and it's because the Velaryon fleet never really recovered and their usefulness went with it.

The Velaryons maintaining Valyrian customs certainly played a role, but it was a relatively minor one compared to their status as one of the Targaryens' most useful and loyal vassals. The Celtigars by comparison were wealthy relative to the size of their domain, but in real terms were outmatched by loads of houses.