It’s like an All-Star team…the individuals might be magnificent, but they don’t necessarily play well together.
A couple of fun Black & Tan facts before I jump into the fray. Generally, when the concoction is referred to as a “Black & Tan”, it’s served as a pousse-café with the dark beer clearly separate from the lighter one. A “Half & Half” is when the lazy bartender just dumps the brews in together, just mixing them together, unless you are actually in Ireland, where they get kind of huffy and blustered if you order a Black and Tan.
The layering is usually achieved by utilizing a funky little spoon with a shallow, round “bowl” and a notched handle that can rest on the lip of the glass. I prefer a tool called a Brutul Turtle that fits over the top of the glass and allows you to pour the dark beer over it and into the lighter beer, keeping them separate. There is a newer tool that has holes in the top. It fits over the top of the glass and when you pour the dark beer over it, the beer layers on top of the lighter beer. Some bartenders just use a bent spoon and pour the dark beer over the rounded end.
First, the “Tan” beer is poured in about halfway into the glass. When the head settles enough, the “Black” is very slowly poured over the back of the spoon. A deft bartender can create a perfect delineation between the two brews (which is, of course, destroyed as soon as you drink). Black & Tans are popular in Ireland where the Irish beer (Guinness) floats on top of the British beer (Bass), thereby symbolizing Irish dominance over the Brits. For those unschooled in history, those two nations have had a “spot of bother” over the past few centuries. Something about religious differences, mass genocide, and explosives. But at least those differences have blessed us with a cool-looking beverage to drink in pubs.
Another worthwhile thing to note about a traditional Black & Tan…the Guinness, or stout, “floats” on the Bass (or Harp), the lighter colored beer, because it’s a lighter-gravity beer. So when a friend scoffs at drinking Guinness because it’s so “heavy”, show them a picture of a Black & Tan and remind them, in the most condescending, patronizing manner possible, that just because a beer “looks” heavy, it does not make it so.