Foveon sensor cameras (from Sigma) are highly regarded by some for BW work, as often you want to use a single channel (e.g. red) for your images (akin to colour filters and BW film) and with a Foveon this means you are still getting one sample per pixel in your final image.

"The Sigma DP1 Merrill makes a superb black and white camera at 1/10 the cost of a Leica M Monochrom + lens."  - Digilloyd.com

With a Bayer or Fuji X trans sensor when doing a single channel BW conversion you either have one sample every 2 pixels (green channel) or one sample every four pixels (red or blue channels). This means your 24MP Bayer sensor acts like a 6MP monochrome sensor when doing a pure red channel BW conversion. A 15MP Foveon yields 15MP.

You can produce a single channel BW conversion post demosaicing (this is perhaps the more common approach) and here the Bayer (or X-trans) sensor can exploit correlations (similarities) between colour channels and so the loss in resolution is not as severe (roughly equivalent half of the pixel count, depending on image content). But this leaves the potential for demosaicing artifacts (which can be quite severe with the X-trans) which do not occur at all with a Foveon sensor.

Whilst a true monochrome sensor (like such as found in the Leica monochrom) seems like the holy grail for BW photography (extra sensitivity, one sample per pixel, no demosaicing) in reality the difference in quality is not that massive, meaning you have to take other factors into account. At the end of the day a camera with a very good RGB image will also yield a good BW image.