According to this account, the occasional "blue ice bomb" is not because of flushing to the outside, but because of leaks in the plumbing that allow a generally small amount of fluid out. At altitude, it freezes as it escapes, forming a block of ice, as the plane descends to warmer temps, it drops the bomb.

All of the non-blue water is theoretically potable, but there have been several accounts of random testing that have found a variety of bacteria and such that would not be healthy to ingest. The tanks are apparently not disinfected on a very regular basis, washing my hands is about the extent of how far I would go with it, certainly not drinking or teeth brushing.