Half an hour is "some time"? And I couldn't find a creature that had bless water as an SLA, though the opposite may be easier to find.
Buying it is not as cost-effective. It's expensive!
Holy and Unholy water are sold at cost. Remember, it takes 25gp of silver and some water to create either one, and each one weighs 1 pound. Maybe shave 2.5g from that with the Diplomacy rules from Complete Adventurer. At least it's cheap to hire some pack animals to carry it around, but that means having pack animals following the party.
From what I gather, the point is using psychic reformation to get the Imp to have the Blessing of the Godless feat? The Cursed Chalice item would need to grant the equivalent of half a year's worth of blessings, three months' worth if using two blessings, and two months' worth if using all three before it was more cost-effective than just outright buying Unholy Water, assuming the price would still be 4300gp of course.