I seen this on many, MANY occasions in LMB when I lived on Texoma. As Flame stated, this seems to be somewhat common in southern waters, I have never seen this in LMB in Kansas. I remember a few on Grand lake o the Cherokees as well..Only pre-spawn to just post-spawn.
I would guess it to be the same mechanism that causes male Crappie to turn black as well..


www.aquaticspecialties.info
May God be with the Overton's Family.