I've lived in the Clarksville area for 13 years, and Blackhorse has always been the first place on my list to take folks who were either visiting, or were new to the area. For my money, their pizza is the best value in the area when it comes to independent pizza "dives"--alathough Blackhorse is FAR from a dive. Besides the standard pizza fare, they also feature specialty pizzas that are absolutely non-traditional, and you can also build your own. The toppings are plentiful and appear to be fresh.
Not only do they have magnificent pizzas, they also have a selection of mouth-watering sandwiches. Again, some of their sandwiches aren't traditional sandwich shop fare, but give 'em a chance. All that I've had have been delicious.
If you have tickets for a show at the Roxy (and if you don't, why don't you???), come early for dinner before the show.
Like any place downtown, parking for Blackhorse is problematic. Be persistent. A visit to Blackhorse is worth it.