I would use a very strong vacuum to pull out the loose particles. Do not wet (I disagree with Peter) as it will cause the particles to go into the wood pores. You might be able to use a sticky eraser or perhaps some masking tape ( masking tape 's adhesive can cause more damage chemically than its worth so touch lightly). Dampen a cloth (do not soak) with lacquer thinner or alcohol and dab the cloth on the surfaces. The graphite particles are being held to the wood by "electrostatic" charges and are extremely difficult to release. Good luck and don't hurry.
Yeah, one would hope that dry graphite was used. In that case, I also would not introduce liquid initially. Worst case, someone may have used oil based graphite such as Lock-Eze in which case doubtful you'll ever remove all traces and staining.