Article on beer color and lager from Beerspotter
This information is from Wikipedia
The visual characteristics that may be observed in a beer are colour, clarity, and nature of the head. Colour is usually imparted by the malts used, notably the adjunct malts added to darker beers, though other ingredients may contribute to the colour of some styles such as fruit beers. Colour intensity can be measured by systems such as EBC, SRM or Lovibond, but this information is rarely given to the public.
Many beers are transparent, but some beers, such as hefeweizen, may be cloudy due to the presence of yeast making them translucent. A third variety is the opaque or near-opaque colour that exists with stouts, porters, schwarzbiers (black beer) and other deeply coloured styles. Thickness and retention of the head and the lace it can leave on the glass, are also factors in a beer’s appearance.
This is the Wiki article. It has a nice chart that show some of the different styles and associated colors.
There has been a move lately to put out black lagers but if your friend knows and likes beer and wants a black beer, they are probably asking for a porter, stout or imperial stout.
Personally, I would go for the Imperial Stout. There are many good ones on the market.
PLEASE NOTE: IF your friend usually drinks any beer with the word “lite” or “light” in the name on a regular basis the above advice is null and void. Buy one of the black lagers by Guinness or Sam Adams.