We get asked this question a lot, most often by people who are new to the hammered dulcimer. They are understandably concerned with getting the combination of options that will make it easiest to learn this new instrument, and the choice of black vs. natural finish on the soundboard seems like a big one.
We build Dusty Strings dulcimers with three different "color" choices. The laminated birch soundboards are a very light color, the solid sapele soundboards are a mid-toned brown, and either one can be finished with black lacquer instead of clear. The amount of contrast is different between the three, as you can see in the picture above, but when you haven't played before, it's difficult to know how important that is.
When we hear this question, it's usually centered around string visibility in particular, since the dulcimer has so many of the gosh-darned things. The steel strings show up very nicely against a black soundboard, and it's easy for a new player to assume that the extra visibility will be the difference between fun and frustration. There are undoubtedly people out there who swear by black soundboards for this reason, but in our experience, most players are actually focusing on the bridge markers rather than the strings when they play. As long as you can orient yourself using the black and white markers, how well you can see the individual strings doesn't actually make much difference in how easy it is to learn and play the dulcimer. Chances are, you'll find yourself "tuning out" the strings themselves as you get used to looking at your instrument.
Having said that, there are a couple of particular situations we've heard about in which a black soundboard does make it easier to play, and you'll have to decide if either of these might apply to you. There are some people who play under bright stage lights or sunlight and find that the shadows cast by the strings on the soundboard are distracting. Having a black soundboard can mitigate that.
We have also heard from a few visually impaired players for whom maximum contrast is very important. For these folks, being able to differentiate a light-colored bridge and white bridge markers from a contrasting black background can make a big difference.
So what do we recommend? Outside of a few specific situations, we pretty much feel that you can choose what you like the looks of best without needing to worry that you'll regret your choice later. For most people, string visibility is not really a factor in learning to play the hammered dulcimer, so why not pick what your eye is drawn to? After all, you deserve an instrument that you're proud to show off!
If you have experience to share, leave a comment below!
P.S. By the way, we have never been able to discern a sound difference between an instrument with black lacquer and a comparable instrument with clear lacquer.
P.P.S. A couple of small additional points are brought up by dulcimer players in this thread on everythingdulcimer.com, so take those into account if you want!