Aui_2000 from PotM Bottom says that you should Soul Ring into Manta on Brood and then get another DPS item before BKB.
His reasoning is that Brood can do so much more with a Manta than with an early BKB since a Brood without any DPS items but with magic immunity is kind of worthless. Whereas with Manta she can break dust, move much faster, do way more damage, etc. etc. Since her job is to split push endlessly, buying magic immunity just to survive ganks is overkill and will waste charges on the BKB as when teamfighting time comes around... she will only have 5 seconds of BKB left on the timer.
Take from that what you will.
Personally I don't even bother buying BKB on her. I go Manta into Heart. Or if the enemy sucks I Manta into Butterfly. I don't really like buying BKB unless I feel I need it. It's never a core in any of my builds. Because honestly, the stats are really kinda crap for the cost. Especially if you're not a strength hero. I guess the trick is to be able to identify when exactly you need a BKB compared to when you can do without.
For example:
- Enemy is Bane, Sniper, Spirit Breaker, Crystal Maiden, Antimage = Why on earth should I waste 4000g on magic immunity? It only stops like... one of them.
- Enemy is Tidehunter, Earthshaker, Leshrac, Shadow Shaman, Drow Ranger = BKB all the way