Black worms can be tricky to maintain. They are very sensitive to warm conditions and I have found even the transport home in the car during the summer can cause them to start dying.
I used to purchase large batches (a pound) and attempted to maintain them in the refridge. I have found you need to keep them in a broad, flat shallow container with very little water (14x8x3 rubbermaid food storage, no lid). you have to check on them frequently to prevent drying out or freezing. Needless to say, I now buy smaller feedings and divide them into all the newt setups. My tank room temperatures never go over 72F degrees (lowest is 66F). The worms pretty much survive for weeks/months. The newts will feed on them for several days. I don't use a substrate (sand or gravel) in the bottom of my tanks, only large smooth stones and rocks. This will keep the worms exposed enough for easy capture. I will lift the stones periodically to exposed hidden worms. I even sometimes save the bottom sludge water after tank cleanings and water changes, to rinse out more worms. If you use gravel, I have placed a shallow dish on the bottom of the tank and fixed a worm dispenser above it, this slows down the migration of the worms into your gravel, giving the newts more time to feed.