The Belgian tourist-board gave me this information about driving with the blue badge:
If you have the European blu-badge card you are allowed to use it in Belgium.
The regulations about the card can be different from town to town. Most local authorities in Belgium allow parking badge holders to park for an unlimited period and/or free of charge, but this is a point that can be different from one town to another. Towns that expect blue badge owners to pay for parking mention this on the parking meter. Of course, this means that it’s not possible to give an answer for the whole of Flanders.
If you want information on the parking policy for a town in particular, feel free to let us know, and we’ll try to find out for you.
The Belgian tourist-board has an excellent site for disabled. If you plan to go to Belgium, either arriving by car or by air, I would higly recommend you to take a look at the page.