Aw hun, what a rough road you're traveling! I'm so sorry you guys are still struggling. It must be even harder knowing it was easier with DD1.

It's hard to say but it doesn't sound like a blocked duct - if it has a blood blister type thing, it might be a problem with her latch, causing a squeeze on your nipple. How is her attachment? Can you remember, before the blister appeared, was it painful when she attached?

You will be able to build up your supply again if you're resting that nipple for a while, try not to worry about it. It might need a bit of a rest to heal if it is too painful to attach DD. Try and pump if you can, that will help maintain your supply. You could try using a nipple shield, that might help?