Aura Sphere and Dragon Pulse have to be bred on it, but Dark Pulse can be taught to Blastoise as a TM. It comes very late as I believe you need Waterfall to get to it on Route 16, but it's there.It's worth it. I have two, personally.
The main problem is that Mega Blastoise's ability is near useless for playthroughs as it can only learn one move that works with it through level up: Water Pulse. To get Aura Sphere and Dark Pulse, you have to breed it.
I would be surprised about Gen V Megas since they did say they wanted only older Pokémon to get Mega Evolutions. True, Diancie is getting one, but that might just be more on a promotional point of view (I also wouldn't be surprised about Mega Zygarde for Pokémon Z. Or Mega Zygarde X and Mega Zygarde Y for Pokémon X2 and Pokémon Y2).