I've noticed this too, on the upright piano I grew up with. There's a definite "thunkiness" to the sound of black keys. I've only played digital pianos since, a

I've noticed this too, on the upright piano I grew up with. There's a definite "thunkiness" to the sound of black keys. I've only played digital pianos since, and haven't noticed it, which is not surprising given that most digital pianos use the same sample for at least 2-3 adjacent pitches. I also do not know the reason, but I suspect it might have to do with body resonances/geometry.