For a permafreeze team (Kaeya (main) + Chong (to make AA ice damage) + XQ/Barb (apply water)), BL will probably be BiS unless you have some amazing artifacts. Kaeya gets so much free CR from the cryo set + resonance + C1 bonuses when enemies are frozen that you're biggest need will be CD in order to maintain an optimum 1 to 2 ratio of CR to CD.
Watch the Jinx video for a full explanation. Basically, the cryo set makes Kaeya with constellations around S-tier with the right team and content that rewards permafreeze. He won't ever beat the pyro Diluc/Klee teams but permafreeze means you get hit a lot less so the run becomes easier.
But there isn't a ton of content that favors an awesome permafreeze team. My understanding is that Abyss is still favors pyro with some geo buff so it's kind of moot right now, but I haven't gotten too deep into 9-12 so I could be wrong. If they keep on tweaking Abyss eventually Kaeya (or a new cryo 5-star probably) will get his/her moment.
Which is insane because he was a freebie.
So it's kinda weird. You can go get BL and farm artifacts to have a top-tier permafreeze team if you have Chong. But you don't really need an awesome permafreeze team right now since everything other than Abyss is easy. Since BL is purchased from the store, it's debatable whether it's worth the investment vs. saving for more constellations into a four-star.
Personally, I'm going to buy BL unless I get Fischl to C5 in the next couple of days from my remaining pulls. I'm slowly building a Kaeya main but he's lower priority than finishing up building my support, plus Fischl C6 > Kaeya permafreeze because I can slot her into a lot more team comps. But I'm at Fischl C4 so if I don't get C5 from this banner, I'll just buy the BL and wait for the next rate up to get Fischl to C6.