While Ethan gives a comprehensive answer to the assumed question “how much does dark matter contribute to the composition of black holes?” the far more interesting question may be: “what effect(s) may be detectable from relatively tiny amounts of dark matter being accelerated to relativistic speeds in consequence of skimming the black hole’s event horizon? We know that most matter pulled toward the gravity well of a black hole will in fact “miss” the event horizon and instead be accelerated to relativistic speeds and thereby show itself by means of various emissions (which is how the first image of a black hole was captured: the baryonic matter around the event horizon “glows” while the black hole itself is of course invisible). So the same acceleration should happen to dark matter and the question therefore is: what theoretical signature would be given off as the highly accelerated dark matter speeds out and occasionally (via gravity) has an effect on the baryonic matter through which it subsequently passes?

This seems to be to be an interesting question even though, as Ethan correctly points out, the actual amount of dark matter involved would be tiny compared to baryonic matter. As such, any effect contributed by dark matter would be undetectable until the various baryonic particles likewise accelerated dropped away, influenced by their electromagnetic and other interactions with other baryonic matter in their path, thus losing energy and speed. At this point, only the dark matter would continue moving away from the black hole at relativistic speeds, and it is at this point that any interaction with baryonic matter would be detectable and not merely ascribable to “ordinary” matter.

The key point is that any theoretical consequence of the phenomenon would permit a way to point to something more than merely the mass deficit that suggests something (which we call for convenience dark matter, but which otherwise is largely undefined) really is out there.