It's no longer like that.
As you see in the following image, I just imported a wallet that has been generated by electrum and has never received any fund into bluewallet successfully.
Hmm, good to know. Did you ever try it before and what would happen? Did Blue Wallet provide an error mentioning that the seed import didn't work or that it was a wrong seed format...?
Additionally, have you checked if both wallets generate the same addresses? I really hope it works and Blue Wallet doesn't produce errors if you went ahead and used that imported (unfunded) Electrum wallet.