Can I clear the mempool from this transaction? I accidentally sent a payment from my blockchain.info address to my Bitcoin Core address with a really small fee (5 satoshi/byte). The transaction is still currently unconfirmed, for 4 days now.

I heard Bitcoin Core has mempool.dat, but I'm unsure if this tactic works only if you sent the TX from Bitcoin Core.

If I delete my mempool.dat, will that send the bitcoin I sent back to me on blockchain.info? I'm sending my bitcoin to my Bitcoin Core address that has the mempool.dat. Let me know. Thanks!
It doesn’t work that way. Your transaction resides in mempools of other nodes of bitcoin network. Mining nodes take the transactions to the block from their own local copies of mempool. Deleting this file on your node will not help you.

You can however send the same transaction but with higher fee: Is it possible to add a miner's fee after you send your bitcoins?