Pure nickel strings don't sound dull. They still produce clarity and harmonic detail, but they emphasize smoother highs and less aggressive upper harmonics. Many players also find they feel slightly smoother and softer under the fingers.
Many blues players prefer pure nickel because of its smooth highs, softer attack and touch-sensitive response, which pairs well with vintage-style amplifiers and pickups.
Nickel-plated steel balances warmth, brightness, attack and clarity across many genres. It's commonly preferred for modern rock and metal.