Is it safe to store a sha256 hash of my BIP39 seed on the blockchain? According to https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki a 12 word BIP39 passphrase has entropy of 128 bits.

My reason for doing so is that when I want check if I memorized my passphrase correctly, I could just check it against the hash instead of having to generate the HD wallet addresses and checking its balance.

So is it safe to store a double sha256 hash of my BIP39 seed on the blockchain?
You could and it's probably not a direct security issue, but it's a senseless waste of time trying to find it again. Keep in mind that doing this destroys the PBKDF2 key stretching used in the seed needlessly. Electrum stores a checksum with their 12 word seed (making it a 13 word seed) if you really desire that functionality, though it's clear if you have it right when the money in that wallet is restored.