I have a "basic" recipe for roast beef that uses 1 bottle of black beer as an ingredient.
I am looking for a non-alcoholic ingredient to replace it because my parents don't drink alcohol and I usually don't have alcohol on hand.
Black Beer is not beer as you would know it! It is a thick concentrated malt liquor. The best substitute would be a non-alcoholic dark malt liquor, or some malt extract.
Try some variety of Malta beverage. Malta is a hopped, malt soda/pop. Essentially it's unfermented and carbonated beer. Especially if black beer is exceptionally malty and sweet, Malta would be a good non-alcoholic substitute. If your recipe has some sweet aspect elsewhere you might cut back on that if Malta is too sweet.