Yes, tsetse flies are famously and aggressively attracted to blue and black colors, including the specific shades of denim. Scientists believe these biting flies, which are found in certain woodland areas of sub-Saharan Africa, perceive dark blue and black as high-contrast targets that resemble the hides of the large animals they typically feed on. In fact, many tsetse fly traps are specifically made using dark blue and black cloth to lure them in. For travelers on safari, wearing denim jeans or dark blue shirts can make you a "magnet" for these flies, which deliver a painful, horsefly-like sting and can carry Sleeping Sickness (though the risk to tourists is very low). The "gold standard" for safari attire—khaki, tan, and olive green—is not just for style; these neutral, light colors are the least attractive to the flies and help you blend into the environment. If you must wear denim, it is highly recommended to stay inside the safari vehicle or use a heavy-duty insect repellent, though many find that the flies are persistent despite traditional sprays.