What is used to give a wood clarinet the black color?

If the clarinet is made of grenedilla wood, then it is naturally that color.

they dye it black to hide imperfections in the wood.

the wood is originally brown

Both replies are partially correct. The wood naturally comes in different shades of dark brown to black. Then manufacturers (not all I'm glad to say) go & hide the natural beauty of the wood with some sort of black stain! A by-product of this is that if the body surface gets damaged it can be difficult to repair it invisibly.

Any way to remove it I wonder?

you could take off all the keys then sand it down to remove the dye, unless your clarinet is made of ebony (like mine is) then there is no point.

Oh no, don't sand it! You'll do nothing but cause damage. Old antique clarinets that have been ignored for centuries sometimes are a very light colour. When a repairman lubricates the wood with bore oil the dry wood absorbs the oil and turns a much darker more vibrant colour. Clarinets that are made out of African blackwood or grenadilla are often very dark if not almost jet black in the first place. Absorbing the protective oil just makes them all the more darker. Yes, I do believe some cheap methods involve stain to acquire the more characteristic black colour, but I personally think a rosewood clarinet is more beautiful than any clarinet painted black just to be black.