OK, I found out more. If you actually mean "Fly Geyser," then the above comments seem to be correct, the land owner may now be charging for entry (and requiring an insurance certificate); however, several geyser gazer friends went years ago without paying anything, the owner then was allowing free visitation.
However, T. Scott Bryan, author of "Geysers of Yellowstone," thought Black Rock Geyser might not be the same as "Fly Geyser." His comments are below:
"I believe that both previous respondents might be wrong, as this name implies that this is NOT the Fly Ranch thing.
"More likely it is a spring (probably the southernmost) in the 5+ mile long Double Hot Springs-Black Rock Hot Springs trend along the west face of the Black Rock Range. As the proverbial bird flies, this is about 45 miles northeast of Gerlach and roughly 25 miles east-northeast of the Fly Ranch area.
"Per Garside and Schilling (1979) (yes, it is getting dated), there are traces of travertine and siliceous sinter among these springs, of which the southernmost had a temperature of 202F, right at boiling for the altitude. That, perhaps, is the Black Rock Geyser."
So there you have it. Either way, it might be good to find someone who has been there recently to get the scoop. Good luck, would love to see your pics of the area!