What kind of a risk do rogue black holes pose to Earth?
almost none, I should think
You might be interested in this technical article
Here's a sample exerpt from the conclusions section near the end:
"With so many small black holes having been ejected
from their host globular clusters, we speculate that ∼
100 rogue black holes are swarming about in the Milky
Way halo with masses from ∼ 100 − 1000M⊙, and with
velocities mostly on the order of a few hundred km s^−1.
The number of rogues could be in the thousands if, as has
been suggested, the current globular cluster population
is a small fraction of the total number originally created
(Aguilar et al. 1988)."
The article has been submitted to the Astrophysical Journal---it is not just a popular science-journalism piece.
Still too early to know if it will pass peer-review and be accepted for publication. I know two of the authors somewhat by name and reputation and my hunch is that it is journal-quality and will appear in ApJ.
the impression I get is that the risk from "rogue" BHs (which I think just means BHs given random eccentric motion by the kick of their formation) is negligible----the risk is vanishingly small and can be ignored.
Separate names with a comma.