I agree with Steve about choosing oak. The end grain exposed by the scrolled cuts will be so vulnerable to weather variations that you will need the best quality, most stable wood available. Quarter-sawn would be my choice even though it is pricey.
No matter how liberally you coat the oak with varnish/oil it will darken over time (I made a sign for my house out of oak, so I know this), presumably due to metals in our polluted air reacting with the wood's tannins. For this reason you might wish to consider choosing a colour that is different from black for your backer.