First off, I’m not much of a Sienna player, although Unchained helped that a bit, so not much experience behind my thoughts here, more just looking at the character.
Inferring from BM’s passives and Talents, she seems to have a two-fold idea: On one hand, getting single big blasts out to kill a lot of enemies at once; on the other, pacing her activity between casting aggressively and defending and cooling down. Unfortunately, neither of these is supported that well.
Bigger trouble, I think, is the pacing approach: There just isn’t time and room for that in the game, especially at higher difficulties. You need to be alert and active constantly, as patrols, roaming enemies and specials try to get you, and with the pace of most (nearly all?) groups, new enemies are encountered constantly. There’s no time to breathe, cool down and let Tranquility do its trick. Unless you let your friends take care of all the random enemies, but I doubt anyone of us is playing the game to take a walk. You could take the ambient folk on melee, but as Tranquility stops if you take damage, it’s a significant risk.
The smaller trouble, if only because it’s (seemingly, at least) easier to affect and fix, is the focus on massive spells. But that is only somewhat supported by her Talents and passives. There’s Pyromantic Surge, which affects all ranged, not just the big blasts; some spell charge hastening (the most significant of which affects only the first spell cast after Tranquility, so quite situational) and… That’s it.
There also doesn’t seem to be much that BW can do that Pyromancer cannot, so BW doesn’t have a niche to fill exactly. Her Career skill doesn’t exactly support her role (either one) either.
So what exactly would make me more interested to play BM? First, find her a clear role to play (that fits the game too). Then, tweak and rework her abilities to clearly support that role. “Caster of huge blasts” is in my opinion a quite valid role, and one she could excel at. “Paced caster”, as I said, isn’t (at least in the current style of cast, wait a while, cast again). Paced caster could work if there was some interplay between her casting and melee, perhaps by dissipating heat through melee hits instead of waiting. Increased ranged charge attack damage at the price of a bit more overheat might reinforce the “Blaster” bit. And finally, rework her career skill to support at least one of her roles.
TL;DR: Her abilities and the general style of the game do not synergise well with her (apparent) role.