My answer comes almost a year after the original question, but I'll post it anyway for those who end up here after a search.
Black & Decker no longer supplies drive belts for the BR400 sander, but you can get a replacement elsewhere if you know what to ask for.
The belt is a synchronous, single-sided "XL" series belt, and mine had 120XL031 printed on it. The belt width is 0.31in or 5/16in or 7.9mm. Its pitch (distance from tooth to tooth) is 0.20in or 1/5in or 5mm. Its total height is 0.09in, and tooth height is 0.045in. It has 60 teeth, making the total length 12in.
I found a replacement at sdp-si.com, part number A 6R 3-060031.
Look
here for timing belts (inch), then narrow the search for single-sided, .200 (XL) pitch, 60 grooves, 5/16 width.
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