Step 1

For The Mash: In a food processor, pulse blackberries, blueberries, peppers, and salt in a food processor until berries release liquid and mixture is coarsely pureed, about 12 to 15 pulses.

Step 2

Transfer mash to wide-mouth, 1-quart canning jar. Tap jar lightly on palm to remove any air pockets. Cover surface of mash with plastic wrap, pressing gently to ensure full contact, and weigh down with fermentation weight or small jar lid. Seal jar with airlock lid following manufacturer's instructions.

Step 3

Store pepper mash in dark area away from sun and let ferment, maintaining an ambient temperature between 55°F (13°C) and 75°F (24°C), for 7 days; check mixture daily for signs of gas formation (mash will climb up sides of jar; this is a good sign). Starting on the 7th day, taste mash daily until it has reached a pleasantly sour flavor; the total fermentation time can take anywhere from 7 to 10 days.

Step 4

To Finish the Hot Sauce: Transfer mash to a blender. Add vinegar and sugar, and blend on high speed until smooth and emulsified, about 1 minute. Measure pH using strips or pH-meter to make sure sauce is about 3.4; if it is too high, lower pH to 3.4 with more vinegar as needed.

Step 5

Strain sauce through a fine-mesh strainer set over a nonreactive bowl or large container, pressing on solids to extract as much liquid as possible; reserve solids for another use (see note).

Step 6

Transfer strained hot sauce to glass bottles or jar, cover, and keep refrigerated until ready to use. The sauce can be refrigerated for up to 1 month (note that because of additional sugars from the fruit, it can continue to ferment as it sits; this shouldn't be an issue under refrigeration, but can cause bubbling if left at room temperature for a while).