My first thought was that BB's devil fruit power can absorb the "ability" of other fruits and keep them isolated within his own. It would be similar to a pocket dimension made by his darkness. If so he could then apply the fruits ability to whatever area of his body at will and then reabsorb it whenever he wants to stop using/switch the power.
The second was that he seems to have an unexplained ability or essence of his own that can be used in conjunction with the yami-yami. Might explain why he wanted that particular fruit.
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