When you're in a region without a matching image (i.e. in the blank sky or uncarved ground), the tile is only a 1x1 image; in Firefox at least, the default image rendering technique for this is bad, as it uses something along the lines of bicubic interpolation to some shade of grey, where what is intended is that it be a solid block of colour.

My solution was to add this to the CSS:

    .leaflet-container img { image-rendering: optimizeSpeed; }

Note also that the 1x1 blank ground tiles (e.g. http://xkcd-static.rent-a-geek.de/converted/4-4-8.png) are #0a0a0a rather than #000000.