
Dark rum is a rum defined by color — deep brown to near-black — which comes from long aging in charred barrels, added caramel, or (often) both. The category is about color, not age, so "dark" says little about time in wood.
Weight and molasses are what it brings, and color alone is easy to fake while weight is not. If the recipe wanted the body, a lighter rum leaves a hole no caramel fills.
Gold / amber rum
Lighter in color, with less molasses and burnt sweetness. In a Dark 'n' Stormy or a grog the drink comes out lighter and thinner: add a barspoon of cane molasses, or pour a longer measure, to get the weight back.
Blackstrap rum
A more intense sub-type of dark rum: heavy on molasses, with a much stronger, almost bitter molasses flavor. A little goes a long way, and a full measure will take over a drink built for plain dark rum.
"Aged" is a claim about time in barrel; "dark" describes color only. A rum can be one without the other, so a dark rum colored with caramel will look the part and leave a drink built for barrel weight noticeably thin.
"dark rum = well-aged / better"
The color is often caramel coloring rather than barrel time, so "dark" means neither "aged" nor "better."
It has cinnamon, vanilla and sugar added. The drink turns sweeter and spiced: drop the syrup from the build, and note that spiced rum gives none of dark rum’s molasses depth.

Dark 'N' Stormy
Dark rum, Ginger beer

Dark 'N' Stormy – IBA
Dark rum, Ginger beer

Dark 'N' Stormy Highball
Dark rum, Ginger beer

Dark 'N' Stormy Shot
Dark rum, Ginger beer

Jungle Bird
Dark rum

Jungle Bird – IBA
White rum

Hurricane (recipe No. 2)
Dark rum, White rum

Bermuda Rum Swizzle
Light gold rum, Dark rum

Black Rum Swizzle
Dark rum

Lost Plane
Dark rum, Aperitivo, Amaro

Three Dots and a Dash – IBA
Agricole

Tiki – IBA
White rum, Dark rum