blackened carrot cake
Two things come to mind 
     1. Do you peel your carrots?
     2. Metal pan reaction .
Try peeling your carrots.  I have gone as far as cleaning my fresh carrots with a green scrub pad. But have found nothing works as well as peeling them. Even if you never peeled them before, when you said you did not have this problem, It could be a reaction to something thats being "added" to the carrots now days.
The other thing I had a problem with in the past was the type of pan I used. I found that some metal pans would discolor my cake. And the longer the cake is left in the pan it would effect the flavor as well.
Hope this helps someone. Have a great day!