Over 60 observations of this hot spring erupting in some fashion over the last 18 years. https://geysertimes.org/geyser.php?id=Black+Diamond Many of the reports mention black water and rocks and "big" -- so not particularly rare. This eruption appears to be larger though -- typically the rocks and debris do not make it to the boardwalk.

They’re not that rare. The USGS statement references a few examples of similar events including one from 2009 in this same basin, one from 1989 in the Norris basin, and another (small) event in the Norris basin earlier this year.