Pulse the black peppercorns in a spice grinder until most of the blend is fine but there are still some coarser pieces.
Add the black pepper to a saucepan with the cider vinegar, white vinegar, sugar, red pepper flakes, salt and garlic. Bring to a slow simmer over medium-low heat and cook until the sauce thickens slightly, about 20 minutes. Use immediately, or refrigerate in a nonreactive container for up to 2 days.
Cook's Note: To use as a mop sauce, dab thick pork chops, ribs or chicken pieces on the bone every 5 to 10 minutes during grilling. This also makes a great BBQ sauce for pulled pork.