The Blue Blazer was the first flaming cocktail. It was made with scotch whiskey and boiling water, which was ignited and passed between two mixing glasses until the fire was extinguished. It was then sweetened with white sugar, garnished with a lemon peel, and served.
Here is a recipe from 1895:
Blue Blazer
Source: Modern American Drinks - How to Mix and Serve All Kinds of Cups and Drinks; Kappeler, George J.; 1895
One lump of sugar dissolved with a little hot water in a mug; add one jigger of Scotch whiskey. Ignite this mixture. Take another mug and pour from one to the other several times, giving it the appearance of a streak of blue fire. Serve in hot-drink glass. Add a piece twisted lemon-peel and a little grated nutmeg.