Twice a day is likely more than they need.
Is the blackish stuff slimy. Does it look like the plant is rotting or is it something dark growing on the plant.
From the picture it does look like some kind of algae and you probably want to remove those infested sections of hornwort.
Also if blinds “usually closed” means they’re sometimes open, lol, the sunlight is likely part of the culprit. Even with blinds closed it likely gets some indirect light. Tanks by windows are very VERY hard to keep algae under control in.