I mount most of my B&W's with Bainbridge Alphamat, matte white, which is slightly bone coloured, or photo white, which looks a little more grey. This depends on the type of paper I use, either warm or neutral tone. For high key prints I will often use a double mat, with black being the inner.
A double mat of the same colour can be quite effective also, but adds considerably to your cost. I find that most galleries around here are very conservative, and just want that off-white mat with a thin black or natural wood frame.
I never have had much success with black mats, but these can be effective with colour. Bainbridge make a grey mat called photo grey, which is as near as damn it to your Kodak grey card, and this can work well with low key prints, especially with an off white inner mat.