There are a variety of different types of Mauls that all fall into different categories.
The Giant black maul is not specific in which category it falls into. Most mauls are in essence warhammers. Therefore, they are clubs. There are, however, splitting mauls which are axes with a large flat side opposite to the ax blade. Maul's can also be like sledgehammers, based on their design.
What is clear, however, is that Mauls are rarely considered to polearms. A polearm, is in essence, a long handled weapon with some type of end. You can have a variety of these as well. Halberds have an ax blade and a spear point. Bec du Corbin is almost identical, except instead of ax blades on both sides it has an ax blade, a spike on top, and a flat bludgeon on the opposite side.
Personally I never have considered that the Giant Black Maul should be anything but a club. If you wish it to be a polearm I would suggest a name change from Giant Black Maul, to Giant Black Morning Star (which consequently are typically considered halberd but are clubs in genesis...go figure.)