I believe the ethereal blooms affect an 8-block radius circle, not a square. I tried placing one in the center of a naturally-spawned tainted chunk, and it did not clean the corners of that chunk. I ended up placing 4 of them; one in the center of each side of the chunk. That did the trick, but it takes a lot of blooms.
After creating six new worlds and finding tainted biomes visible from spawn on all six of them (MW2), I just disabled naturally-spawned taint biomes in the config. I can still taint the land if I'm not careful, but starting out with a significant portion of the "spawn chunk" as tainted just sucks. I wonder if I can re-enable them after I load the spawn area?