What's the difference between xpub and bpub?
bpub appears to be the extended private key prefix for the Blockcypher testnet.

Bitcoin addresses and private keys usually have a prefix to help differentiate the network/type of key.

The original xpub notation comes from BIP32, meaning extended public key. This expanded into ypub with BIP49, and finally zpub with BIP49+ (used in BIP84).

Various coins can define their own prefix by simply altering the bytes. Litecoin, for instance, often uses Ltub.

It works the same way as changing the address prefix, except you change four bytes, instead of one.