I have built a few boardbows from hickory and several have had a dose of dark heartwood in them. I am usually more concerned with how the grain runs out on the four corners of the wood. From the view of the end grain, I would guess this may be a tough bow to pull out of the stock. Remember, every grain run out on the back of the bow is another strike against your odds of survival.
Ultimately, since you will have not much more than time invested in the bow, go for it! Good luck!