Hard to comment without party comp context, but if we're talking current optimized ceiling Blue Flame Blade is probably the numerically strongest at-will in the game. I think kitsune as a gameplay choice is worth it for that one ability alone. You kill dark mode Eyslaine in one Venom Strike crit with no other damage, through his 60% damage reduction, frequently before your Abjure ends (so he can't even Unbreakable to save himself). The fact that Blue Flame Blade looks at your current spellpower means you get to double dip effects like Blessing and Flames Within on every weapon skill you use. This increased buffability is what made spellblade-style setups overperform in the past, but because the current suite of powercrept endgame powers doesn't include many with both spell and attack power scaling (I think Tempest Smite is the only one?) Blue Flame Blade kitsune are currently the only ones able to get the best buff efficiency on the best powers.
Relatedly, Blue Flame Blade is also set up to exploit the above without sacrificing any of your own ability slots because the necessary synergy happens to be available in some of the strongest companions anyway. The most powerful perks in the game as we know are Expert Inspiration doubling effects like Blessing and Eye for Weakness doubling effects like stuns, and your own turn economy benefits enormously by having Etheryn doing the Blessing for you so you can do giant, flaming Whirlwind/Fury Strike/Venom Strike/whatever crits on guys Viviane has Greased. Elementalist Viviane in particular synergizes so well, between the multi-turn stun that also boosts your BFB damage, to Baffle making up for any crit you're missing as well as making sure Twist the Knife is always on, to having a weapon buff ultimate for you the weapon power character, that I think there's no non-rp justification for a kitsune PC ever not to bring her. You even get the 10 spellpower sleep buff to make your BFB just a little spicier.
