how to delete a DM to an account you don't follow?

so i have a second account that i use to post edits, but i don't want any of my friends to know about it. when i'm on that second account and i go to DM a video to someone, all my friends' accounts (that i follow from my main account) still show up. i just accidentally sent a video to my friend from my editing account, but since we don't follow each other it's just in "message requests", and i can't seem to delete/unsend the message. does anyone know if they'll still be able to see the DM if i block their account?