The Canton Domestic is, I believe, the deepest cobalt jar known.  Under casual circumstances without a fair amount of light behind it, it can appear almost black.  It's one of my favorite jars and this is part of the reason.

It has a cousin, the Canton Electric jar which also comes in cobalt blue, although for some reason, in a much less intense cobalt.  I always have to catch myself and remember that the one you'd intuitively think of as the more intense one, the Elesctric, is actually the less intense and the one you'd think of as more...well, domestic, namely the Domestic jar is actually the more intense.  The Electric also normally comes with a clear lid which makes it even that much less interesting.

This example came from the collection of Alex Kerr.