OMFG, you have got to be kidding me! I don't' know where to begin other than with sarcasm
Dark coils sound better folks! Be sure to ruin the driver before you re-cone it!
Dude seriously, I have reconed A LOT (metric assload) of speakers in my life and the ones with darkened coils are ABUSED, some never even go dark even after being repeatedly HAMMERED into the backplate. you just did nothing but possibly weakened the glue bond of that coil.
I should so be in my jammies now but this has intrigued me to the point of no return.
When that speaker is assembled it is moving up and down, this is bathing the coil in "hopefully" a pumping action of cool air, thus cooling the coil. The reactance of the magnet also plays a very important role in the electrical characteristics.
Although I absolutely DO NOT believe in breaking in subs except for testing because they will break in on their own quickly. The point is to utilize the suspension so that it will "rest" to the mechanical properties that it will be at for the rest of it's life (unlikely, but hopefully)
The voice coil SHOULD NEVER change it's properties, as it's not intended to do so. For example, if that were a JBL professional driver, you just voided the warranty