It might be worth taking a look at the Particle Spiral Effect article at Unify Community. The author cites that it "might be used for portals, whirlpools, galaxies, etc."

The screenshot examples he's provided might be a bit unrealistic for what you're after, but I'm sure you could tune the parameters to achieve your desired result.

I'm only just starting out myself, but I think you'd need to use several emitters positioned at the outer edge of your black hole and then you could have them rotate around the centre point (parent them to a single gameobject for your black hole and spin it with constantforce). Then you could just have them emit particles, which die once they reach the centre, in a straight line and the degree of torque applied by the constantforce would control how 'tight' your spiral would be.

Ideally, you'd be able to do this by setting the size for the viewing axis to 0, min emitter range to the size of the other two axes, and using negative values for the velocity over those two axes, but that's not possible since a 2D emitter still counts its third dimension (spawning particles within the min range) and ignores spin upon its own axis. The solution above isn't as elegant, but it should still work.