What's the difference between xpub and bpub? I've seen extended public keys such as the one below

xpub661MyMwAqRbcFLCsW67CH9AgprkK2MbeLbmdMiBRWb1Wamm6GjGhTFQChvPmwQGQh78MMd8Eu7CLJ7FEuwhsomD66XC74VnfgHdEx1NFSKL


But now I've seen a bpub

bpubgqxJY83YqBS1rgM392MqkPnaa7P8yBEkZ7pZksTUHjRxQRn8RzuL8ST1NYhAqgbM7f3SDzTTJifL1WuCYvVxHinLuhF4kFdnBNN34N4Y2et


What's the difference?
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.