If the current passing is severe, it will definitely discolor the oil. In some cases carbon is created.
Have seen dirty cooling tower water passing thru copper tubes cause a jelly like substance to buildup and greatly affect cooling ability of exchanger.
The links also discuss the swap of metal, so depending on materials and their charge, you could be moving tin/lead/copper from motor bearing to the slinger or some other component, temp probe, etc..
Here are some additional resources
How do you know it is electrical current passing thru bearing, could the new bearing be starting to show wear? Is your shaft damaged? Bearing clearance between shaft and bearing too tight?