Black Bay is definitely a good place to go looking for some of those giant crappie Rainy is known for. It is also a treacherous piece of water to try and navigate at high speeds so do use extreme caution when getting off the main navigation routes leading into and out of the bay.
Here’s a previous thread on this topic worth checking out. It is related to fall crappie but for the most part the crappie are doing similar things in the spring… in very similar areas.
https://www.in-depthoutdoors.com/community/forums/topic/rainy-lake-black-bay-crappies/
I’ve caught crappie on the shallow rock piles that are sprinkled around in the back of the bay as well as caught some dandies out of the bull rushes. The crappie around the rock piles and rushes bite well on calm, warm days and can be incredibly difficult to catch in the shallows if the temps drop or you get a stiff wind.
I like to cast a small bright plastic on a light jig head and swim that back to the boat while others do the bobber and minnow thing. Both work.