Could it be also possible to import seed to wallet apps like mycelium? So there would be no need to download a wallet to PC. And on mycelium signing messages is very easy. But I didn't try it out yet, because I don't use blockchain anymore.
If I'm not wrong, mycelium also uses BIP39 for their seed so you should be able to import it although it requires you to have a fresh start first. Meaning to say, if you are already using it of course with an account/s in it, there will be no option where you will be able to import another wallet through a seed unless you had cleared the data. Remember when you first opened the app, it asked you to create a new wallet or restore from another right? It should revert to that state.
If you insist on signing a message through mycelium, you can use the tool I mentioned in my previous post and import it there.
Since we're talking about importing you can just use electrum as the process will be more easier compared to mycelium although I don't recommend importing just for signing a message since the access to your funds will be available to multiple sides (Based on the number of wallet/s you import to).
Tutorial: https://bitcoinelectrum.com/restoring-your-standard-wallet-from-seed
*When electrum prompts you to enter your seed, Click options and tick the BIP39 Seed Box
*When electrum prompts you to enter your seed, Click options and tick the BIP39 Seed Box
