Tar darkens wood because it stains the surface fibres with dark oily compounds and forms a black or brown coating over the grain.
Wood darkens quickly when tar reaches the fibres. The colour is not only a film sitting above the wood. Some of the tar moves into surface pores, checks, and end grain, leaving a stain that can be difficult or impossible to remove completely.
The darkness depends on the tar and the wood. Pine tar can give warm brown, red-brown, or black-brown tones when thin. Heavier tars and repeated coats can make timber nearly black. Pale softwoods show the change more dramatically than already dark hardwoods.
Age changes the colour again. A fresh tarred board may look glossy and rich, then become flatter as it weathers. Sun and rain can produce greyer or smokier tones, while fresh recoating restores a darker finish.
This darkening is one reason tar was used as both protection and appearance on rural buildings, fences, boats, and tools. It is also why test patches matter: tar does not behave like a clear finish and should be expected to alter the character of the wood.