This is a DELTA shower cartridge. The blue color is a key to the model. Readily available at Home Depot. Here is how to replace:
Take your original with you. If you hae trouble, I have found the DELTA customer service people to be extremely helpful. Just give them a call: (800) 864-2555
GOOD LUCK .... and if the hot/cold does not work correctly, you installed the cartridge upside down. Remove it, rotate it, reinstall. Easy mistake to do.