You can also use those spray cans of rubber for cars that they use to completely recolour the car. I can't remember what they are called, but they will give you a matt finish and you can peel it off later if you decide you don't want it any more.
Blackboard Paint, it will stay in-place under normal circumstances but it's increadably easy to scrap off if you ever change your mind!
Matt paint will always have a gloss finish as it's surface is visible through the glass. For a Matt finish you'd need to apply the paint to the internal surface of your aquarium!