I'm done with blastos too after experiencing the same thing a few times. I'm convinced it has something to do with metal halide lighting over the long haul. I think a lot of blastos, even though they seem to thrive in the short term, deteriorate over the course of months or even a few years until they receed into nothingness under MH. I've never been able to keep blastos under my MH for more than a couple years at the most. They do survive in my refugium lit by PC, however they grow and multiply at an extremely slow rate. I've saved many that started to receed simply by moving them into the fuge. Interestingly, my MH is not at all intense - 1 250 watt 15K XM over a 36" x 18" x 20" tank, flanked by 2 96 watt actinic PC's and there are plenty of shady spots in the tank. The fuge is a 40 gallon breeder tank with 2 96 watt PC's. I think current is over rated as a limiting factor in blasto health specifically and with most LPS in general as long as it's not so strong that it damages flesh. The problems I've had with unexplained die offs were with Wellsis for the most part. I've had much better success with Merlettis although they are not exactly problem free either. Something to think about - how many huge blasto colonies have you seen? I could be wrong but I don't remember ever seeing one?? I'd love to see a pic of large colony if anyone has one.
). They could be doing well for a while and then start to recede. 
