Here's a simple technique for producing striking black and white images in Lightroom. Make a virtual copy of your color image, convert to black and white using the one-click setting under the Basic tab of the Develop module, and ... here's the magic ... play with the color temperature sliders, both TEMP and TINT. These sliders change the underlying color mix of your image and, in so doing, the contribution the three color channels make to the black and white version.
Some adjustments will darken or lighten the overall image, and you can then play with the exposure and contrast sliders. Finally, of course, you can use the B&W tab to further tune the filtration. But if you've moved one of the color temperature sliders radically, you may find that moving the black and white mix has little effect. (If your color temperature is far to the warm side, moving the blue slider will have little effect, since there's so little of the blue channel in the image.)
This may or may not work on a particular image, but remember that Lightroom adjustments are non-destructive, so you can always move back through your history tab to find a state that worked ... or start from scratch. The following image of rapids along the Mackenzie River in Oregon is one that used this technique, plus a small boost of exposure in the foliage at upper left using the adjustment brush.