The answer lies in the cognate "coaler."

Some etymologies say that the origin of the words "collier" and "colliery" lay in both coal-gathering and charcoal burning, so the prevalence in the Forest of "colliers" and "collieries", rather than mines or pits, may be explained by the considerable amount of charcoal burning that historically went on in an afforested area beneath which lay a considerable amount of coal.

The connotation is blackness, a colley being a black-faced sheep and the commonest collie dogs being substantially black. In some parts of the West Country, blackbirds have been known as "colleys" (which in turn may explain the variant in "The Twelve Days of Christmas" between "colley birds" and "calling birds.")

As Slowhands says, a coal-carrying boat is sometimes called a collier. It is also sometimes called a coaler.

Thanks to everyone for the information!

And sometimes (in C17th) "stone collier" and "wood collier" are used to differentiate between a coal miner and a charcoal burner.