Stir rhubarb into a large pot of boiling water and cook for 10 seconds. Quickly drain rhubarb and rinse with cold water until cool; transfer rhubarb to a large bowl.

Place red onion, green, red, and yellow bell peppers, and cilantro into a food processor and pulse 3 or 4 times to finely chop; transfer pepper mixture to bowl with rhubarb. Stir in tomatoes.

Dissolve brown sugar in Key lime juice in a bowl; lightly stir lime juice mixture into rhubarb mixture. Sprinkle salsa with salt, garlic powder, and black pepper and stir salsa again.