Some triturus juveniles will happily go into a shallow dish to eat blackworms. Some of the smaller more "hydrophobic" triturus might not (I'm not sure). Another food to consider is fly maggots, or maybe a dish of chopped earthworms.
You might do better with a shallow container instead of the deli cup. I wrote this up for an article I'm working on:
Here are the things I've learned about blackworms over the years:
1. Keep cold (fridge). (I have left them out overnight, though, and they were OK.)
2. Don't drown them (water should just barely cover them AND they should not be too deep in the container).
3. Rinse daily with cold (fridged) dechlor water.
4. Once some of them start to die, it's an exponential process (hopeless).
5. Find out what day of the week they arrive fresh to the petstore, and only buy fresh ones. Pet stores do not rinse/store them properly. They will often try to sell you their older ones, unless you specifically request the freshest ones. Also, in hot weather, their fresh ones may arrive in poor condition sometimes.
I keep them alive and healthy in the fridge for a month, maybe longer. They have no smell. I was able to do the same with the ones I used to buy at a pet shop, but only if I got good fresh ones. Hope this helps!