That's a very bad idea to use Blockchain instead of a real wallet where you would own for real your bictoins. To be back on topic, that's surely the fee that Blockchain takes for themselves.
I was just thinking that myself, I have seen couple of gambling sites do that, they deduct 20k Sats fee per withdrawal but add only 10k so they save 10k per withdrawal, not sure if Blockchain wallet has started doing that too but it could be the case.
OP, a suggestion, take your private keys out of blockchain wallet, import them into a desktop wallet like Electrum for ex. and then push the tx with fee it suggest with 50k Sats/KB.