According to the Harry Potter films, in front of a person a Boggart takes the appearance of the thing that a person is most afraid of. What if the person is afraid of boggarts? Would it solve the problem of no one ever actually seeing what a boggart looks like? Although it wouldn't matter for the rest of the world because they will continue to see what haunts them.
If someone is afraid of boggarts and since nobody knows what exactly a boggart look like, it is possible that boggart will take the shape as they imagine. If you assume it to be a fearing creature, it will be the same.
If you have no image in mind, then the boggart will be invisible to you.
There have been debate over this topic across the internet. Here are some:
If they truly had no image in their mind, they would see nothing. But of course, nothing would not be scary to them, so the boggart would cease to be their number one fear. For someone to be scared of a boggart, they must be worried about some aspect of the creature. Since nobody has ever seen a boggart, fearing a boggart's true form is an impossibility. Nobody can fear what they can't contextualize.
Whatever your mind pictured their true form to be. They don't have a corporeal form; they're a non-being that feeds off your fear. Once you're tapped out it sort of drifts off and takes a nap until the next walking dinner plate comes along.
It would take the form of how you imagine a boggart to look, I’d say.
However, I couldn't find any official source for that. But all the links I have mentioned above agree that a boggart will take the shape you imagine.
I would guess it will keep it's regular appearance. It has one, since Moody can recognize one through the wall in the 5th book, and it was not transformed since it was not seeing Moody atm.
Check the discussion at scifi. However, there doesn't seem to be some official answer to a boggart's true shape.