@Alexa ale, Over 3 years in the forum and still don't know to sign a message or follow simple instructions 
Anyways, don't worry about not having imported address in Blockchain.com. If you follow hosseinimr93's instruction quoted below, you might be able to export your seed phrase from Blockchain.com to Electrum and then sign using Electrum wallet
Once you have imported the seed phrase to Electrum, use this tutorial to sign a message using Electrum - https://bitcointalk.org/index.php?topic=990345.0#post_electrum198
Anyways, don't worry about not having imported address in Blockchain.com. If you follow hosseinimr93's instruction quoted below, you might be able to export your seed phrase from Blockchain.com to Electrum and then sign using Electrum wallet
To OP:
You should extract your seed phrase and import it into another wallet like Electrum. Then you can sign message from that address.
To do so, go to "security center". Scroll down and click on "back up" button. Blockchain wallet will give you a 12-word phrase.
Then install Electrum. Create a new wallet and select "I already have a seed" and enter that 12-word phrase.
Note:
1. Download Electrum only from the official website. (https://electrum.org/)
2. Don't forget to check BIP39 when entering the phrase.
3. Select Legacy as your address type.
You should extract your seed phrase and import it into another wallet like Electrum. Then you can sign message from that address.
To do so, go to "security center". Scroll down and click on "back up" button. Blockchain wallet will give you a 12-word phrase.
Then install Electrum. Create a new wallet and select "I already have a seed" and enter that 12-word phrase.
Note:
1. Download Electrum only from the official website. (https://electrum.org/)
2. Don't forget to check BIP39 when entering the phrase.
3. Select Legacy as your address type.
Once you have imported the seed phrase to Electrum, use this tutorial to sign a message using Electrum - https://bitcointalk.org/index.php?topic=990345.0#post_electrum198