What is the difference between these ale types?

It depends on the grains that are used in the brewing process. But a good ale is one that has balance and tastes good. That’s the only criteria that really matters.

Colour doesn’t mean too much in and of itself. I tend to like darker brews, but that’s because I like it to be heavy, bitter, and strong. Those tend to be on the dark side. Guinness, Spaten Ultimator, Newcastle Werewolf, Terrapin’s Moo Hoo, etc.

Now, there is a style of a brew known as a Black IPA (India Pale Ale). They can be anything from a deep brown to very black. I’ve had one or two, but I’m not a huge fan of IPAs

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.

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Optimator. Not Ultimator. Damned autocorrect.

Great thanks guys. I ended up getting dark ale but next time I won’t spend twenty minutes examining and debating every label.

There is a lot of information about visual characteristics, thickness and retention of the head, lace, etc, but the most important characteristic of a beer, in my mind, is overall flavor. I’m not sure if this is a standard description, but for me flavor consists of taste (tongue), aroma (both through nose and through the mouth), the way it feels in the mouth, and the way it goes down the throat.