As noted every program is different. A few of my good friends are wrapping up their PhDs in engineering and I know one of them picked up a MS along the way; I believe he said he got one B in grad school.
In my wife's program (PhD) they either get an A or fail, so a B isn't good enough.
I am in a MBA program and after talking with some of the JD students who are also doing their MBA, it seems as if it is more common in professional degree programs to see a variety of grades (i.e. Cs thru As).
My take on your situation is that there isn't much you can do now about your grade, but going forward I would make certain that is the only B you get in grad school.
Also, as a final note, those that say a B in engineering is "better" than a B in another program is maybe only accurate for undergraduate programs. You can damn well be sure there are tons of people getting their engineering graduate degrees who got straight As and those are the people you will be competing with. It doesn't matter one iota at all how that B "compares" to another program of study.