This is why many of us take photos/videos of the car prior to rentals and/or do a walkaround, as well as insist on noting everything to gate agents upon leaving the lot, no matter how trivial. You admitted you did neither of things. Unfortunately there is no way to prove that you did not do the damage (I am not saying you did it, I believe you, but National can't prove that), so National (and any agency, really) will always go to the last renter of the car when assessing these types of claims. The comments of the return agent and/or her opinion of when the damage took place bear no relevance to the analysis.
$150 is relatively minor though compared with what it could have been, sounds like you got off pretty easy. Pay it and chalk it up to a good lesson. Or don't, and you'll probably find yourself on the DNR list and the claim handed over to a collection agency.
PS: Go blue!
