Sorry, I forgot to respond. I will check the other parameters. Would doing a water change help?If you don't want to take my word for it that's fine. Never hurts to ask. Check out this thread. Particularly the last post.
It would help more though if you could post all the other parameters and light levels etc. Blastos I find like pretty low light with moderate flow and prefer to be fed regularly.
The prime is using the prime mount and is the maximum height off the tank. I have fed the corals in the tank once.How far above the tank is the AI Prime? That's quite a bit of power on a 10g tank. The AI Prime needs to be higher than most lights to get the spread. 12" off the water roughly. It can be pretty bright in the center under the light.
Hard to say on the flow. Location and direction of the water currents can impact the flow quite a bit so it's not just about how many gph your return pump and powerhead are moving. In my experience too much light and/or too much flow and Blastos don't expand like they should. A full tank photo might help to understand better the rock structure, powerhead, and light position in relation to the Blasto.
Based on the current information you have provided, I would be looking for a spot that only receives gentle water currents and is off on the side out of the intense light, or perhaps better yet, partially shaded by some rock work. I would also be interested in your nutrient levels and any feeding the corals received. I don't think it's a water parameter issue if your alk is at 12 and calcium around 500.