My 11 month old has a blister on his gums. It's not directly under any teeth, it's white hard (but not a tooth), and red around it. What could it be?

Teething or Chewing . 11 month old can have erupting teeth, a gum cyst, trauma from chewing on some object they put in their mouth, a primary case of herpes infection, or rarely a canker sore. See your pediatrician to get the proper diagnosis.
New tooth coming in. Your baby most likely is about to get a new tooth in that spot. Before the tooth emerges, the gum may feel hard and look white as you described. Sometimes you can feel the edge of the tooth by rubbing your(clean) finger on the gum. Rubbing and cold on the gum should help it feel better until the tooth erupts.
Tooth erupting. It most likely is one of the primary teeth erupting.