There isn't a car rental agency right at the Boardwalk. There is one over at the Dolphin though. If the Boardwalk concierge is able to do a car rental for you, you would have to go somewhere else to pick it up or have it delivered to you from somewhere. There is a car rental agency on-site at WDW but it is by MK. If you are staying on-site, I think think they will provide complimentary transportation to the Disney Car Care center.
We've used the rental company at the Dolphin when we were staying at the Boardwalk and it was really easy. They have the cars right there on site.
Have a great trip!