This issue was discussed at length in Chapter 4 and rather than repeat ourselves, we'll briefly summarize here. Both MBBs and Black Belts need to have a combination of leadership skills, technical skills, and “soft” skills (communication, mentoring, and so on). Black Belts in particular should also have good subject-matter knowledge of the processes they are improving.
Some key attributes to look for in Black Belts are:
Technical leader in the area of the project
Helpful for the first project
Less important for subsequent projects
Respected by the organization
Analytical thinker— not afraid of numbers
Comfortable with basic statistics
Able to lead teams— ...