Add the honey to a small pot and put it on medium heat. Warm the honey through until the colour changes to a deep amber, about 6 minutes. Add the vinegar and let simmer, swirling the pot occasionally, until reduced by half. This'll be another 5 to 10 minutes. Pour in the blackberry liquid and continue simmering until reduced by half again, about 15 minutes. Keep an eye on the pot as it may bubble over. Stir in a pinch of salt and a grind or two of black pepper, and set the gastrique aside.