My blackaria has almost 40k hp and she's still a miracle worker in arena. She can keep everyone alive for quite a while as well as well as revive one person. She doesn't do either of those when she dies instantly, you're only taking advantage of one of her abilities. There's plenty of teams that take more than 8 seconds to kill, and a quick dying Blackaria will be a detriment there.
Even with 40k hp mine still usually dies before others. So it's hard for me to say it's better or worse, but it's definitely not terrible, and i can take on other Blackaria teams with no problem
There's very few teams that can't be crippled in 8 seconds in Arena. My BW and BS just unload for 8 seconds and people drop. I avoid teams with ultimate BS or too many healers. I've yet to lose a single match when I do that. My losses have come from the double challenge and going up against teams with Shabelle and Blackaria and/or priest. They just survive forever. But I do feel like I'm wasting her just using her as a one trick pony. After I reach about 7000 arena score I will level her up. I already have the gear waiting for her.