Mustard based BBQ sauces are pretty traditional and very popular in the central areas of South Carolina. They are generally served in almost all bbq places in all of South Carolina. You can also find bottled mustard based sauces in grocery stores and are sometimes called "Carolina Gold". The recipes are many and varied.
My basic recipe is yellow mustard, cider vinegar, brown sugar or honey, salt pepper, Worcestershire sauce, sometimes a dash of liquid smoke. Some people put hot sauce or cayenne in theirs. I start with 1/2 to 3/4 cup mustard and add a tablespoon or so of apple cider vinegar, 1/4 cup brown sugar, 2 tsp worcestershire, put all in saucepan heat gently and then just add more of any ingredient until it tastes like I want. Some people add honey, or cayenne. It is really good on grilled chicken, but put it on at the end because the sugar will burn. Also perfect on pulled pork and ribs.
A google search of mustard based bbq sauce recipes will turn up a lot of recipes. Very easy recipe to get just how you like it.
Also check out Alabama White sauce which is Alabama's BBQ sauce. It is pretty good and a different taste.