Fret Doctor.
http://www.beafifer.com/boredoctor.htmDon't be put off by the hokey looking website, it's good stuff, much better than the lemon-scented mineral oils sold as "lemon oil".
It's not a stain, so it doesn't actually
colour your board, but it does make it look darker, especially if it's a dry, open-pored piece of rosewood. On the other hand I've got one LP with a very light-coloured but close-grained board (looks almost like pau ferro), and the Fret Doctor hardly changes the colour of that at all.
If you wanted to permanently
stain your boards - to look like ebony, for example - there are products available for that too. Try the Stewart-MacDonald website.