Still Tasty says rotten tuna smells nasty and is slimy. Bad tuna may seem dull, but it oxidises fast, which may also diminish its look.
Dark tuna cores are due to myoglobin, not rotting (via MyRecipes).
Myoglobin helps strong-swimming fish like tuna conserve oxygen, says naturalist Bruce Collett.
Myoglobin may make the tuna's flesh seem darker, but it shouldn't affect the taste.
However, it may be stronger in that area of the fillet. Foodiosity adds that redder tuna (like Atlantic bluefin) is more delicious than lighter types (like Albacore).
You may now prepare teriyaki tuna and mushroom skewers or tuna escabeche safely.