"Something dark" pretty much by definition is something that doesn't reflect light. How you define "black" depends on the context and just how stringent you want to get with the definition. If you really want to get technical, you might not even say that black holes fully absorb 100% of light. It's true that black holes will effectively absorb all radiation where the wavelength of the radiation is smaller than the diameter of the event horizon (which will typically cover not only light visible to us, but also infrared, ultraviolet, x-rays, radio waves, etc). But black holes will also spit out random photons that are able to escape the event horizon - look up Hawking Radiation. This actually shrinks the black hole each time this happens. So technically, if the black hole doesn't keep getting fed with new energy, it would eventually shrink down and collapse. So, in the (very) long run, even black holes can't be said to 100% absorb light forever!