You could add some smashed gloves of garlic to the brine. An herb of your choice i.e., I'm thinking rosemary would be interesting. I also put lemons in my brine - just cut in half, squeeze, and throw the halves in there too.
As far as adding anything to the brine when you blacken chicken - I can't think of a thing that would help except maybe some peppercorns.
Now that I've said all this I don't know how much all these extra ingredients will actually add to the flavor. Chicken breasts just aren't brined for that long.
When I do a whole turkey I will brine for a good 18 or so hours. It tends to pick up on all the flavors I put in.
IMHO - there's my 3 1/2 cents.