B16 would be a taper arbor. Some drills that are made in Europe or for the European market actually use tapered arbors. The exception to this is drills with a hammering action which usually use a threaded arbor.
The problem with Black and Decker is that they're an international corporation, that produces tools for a whole bunch of different markets, and the tools they produce may be made by them, or for them by other manufacturers, and the specs and standards the tools are made to may reflect the standards in the markets were the tools will be sold, or possibly in the country were the tools were manufactured. The tools may also be made to different specs for different markets but keep the same model numbers. Since the higher end Black and Decker tools are now sold as Dewalt the same problems can turn up with Dewalt tools.
To the OP, as the other members have suggested, first open the chuck jaws as wide as possible and check for a retaining screw. If there is a retaining screw, it may be hex head, or slotted, or possibly something else. The retaining screws usually aren't that hard to remove, but I've never tried to remove one from that well used a drill.
If the drill arbor is threaded, these can be on very tight, or possibly even have thread-locker applied. When I switched the chuck on my Milwaukee, I almost tore my arm out of its socket before the chuck popped loose. I did this by chucking a large L hex key and ran the drill in reverse. The drills cord wound up winding around it too. The method wasn't the safest but it did get the chuck loose. You could also try the suggested method, which is to chuck a large hex key and quickly whack the end with a hammer or mallet to try to free the chuck, this might also work with a tapered arbor. If you do this just make sure to hit it so the chuck turns in a counterclockwise direction.
