Filling with something slightly darker than your wood appears less obtrusive somehow. So Richardwoodhead's advice to add a small amount of black oxide would be good for this. Make it look like sap line.

If this were a woodturning, a few years ago you'd have been in fashion adding metallic chips to the epoxy or coloring it turquoise!

But ... if the cracks are small, then CA with instant application of the ROS with 120 paper will give you a reasonable and strong filler.