I am probably wrong here as usual but my seed phrase being in my spectrum wallet I cannot see how that is a problem, from what I have heard it is safer, that is the only time ever I tried to use it.
It is only safe when you can use it in a safe way. Everything on the internet in vulnerable. So if you choose to keep your assets safe, keep them in personal hardware wallet. The way you expressed your story, I think given the reputation of what blockchain.com have, it is more likely the hacker got the access to your wallet from that. And they can get your seed if they have access over your wallet. No big deal. I am not defending electrum, as it is an online wallet, it could get hacked too.
Lesson learned. Now I think you know what to do and how to keep your assets safe.