In the Stone Be'lal had no reason to conceal his skill with the sword since he wasn't trying to conceal his identity, but instead trying to goad (and probe) LTT/Rand. If, however, he is Taim (or anyone else Rand's encountered) he'd thus have every reason to downplay and deny the same swordsmanship abilities he bragged about to Rand.

It's entirely possible that Taim is simply a new Forsaken (since the symbols he's using are employed not by the Shadow in general or the DO specifically, but the Forsaken and only the Forsaken) but being Be'lal would explain his livery and a lot of other things. The biggest fly in the ointment there isn't Be'lals death by balefire, it's Bashere recognizing Taim and the implausibility of Be'lal employing a Mask of Mirrors to masquerade as Taim. He could simply have used Compulsion to MAKE Bashere remember him as Taim, but there are undoubtedly plenty of other people still around who'd recognize Taim and it's very unlikely Be'lal could hunt them all down and Compel to prevent exposure. If Taim's not Be'lal I imagine he's just Taim, because he seems far too stable and coherent to be Ishamael, who's become increasingly erratic and disjointed over time.