In the context of diabetes BM stands for Bohereinger Manheim. This was one of the first pharmaceutical companies (German) to produce test strips for glucose monitoring. The company was taken over by Roche a few years ago.
To be technically correct "BM's" don't exist anymore (as Bohereinger is now Roche) and we should refer to them as CBGs (capillary blood glucose) however BM is such a common term and everyone understands what it means we continue to refer to CBGs as BMs.