How can I brew a black lager? Is a black lager really a lager (in terms of yeast and brewing time)? Can I just throw in whatever would make a good stout but use lager yeast? If not, then how much play is there to the recipe?

I've got a little bit of experience brewing ales, mainly stout and brown ale. Just a few months ago I tried my first experiment, deviating a little from a recipe in my choice of malts and yeast, but still using a malt extract for my primary malt. I have zero experience with brewing lager.
A black lager, or perhaps a schwarzbier, really is a lager.  Aside from the difference in the fermentation process, there will be some difference in your base ingredients, as a schwarzbier will focus less on the chocolates or coffee flavor of a stout.

I suspect you may find significantly more information in homebrew.stackexchange.com, as hunse suggests.  We would love to go into all sorts of detail over there!