I would assume that combat-oriented or not, most robots of the QCverse are still designed with human interaction and understanding in mind. Hence why they're given personalities, humanoid bodies, and human-like traits. Blushing is a very strong indicator of human emotion, so it makes sense for the robots to have their own form of it. If Bubbles' design was 100% combat-focused, I doubt she'd have arms and legs, more likely she'd be built more literally like a tank.