Did seregon grow only on Amon Rûdh?
That is a possibility. Seregon’s entry at the end of the Silmarillion reads “'Blood of Stone', a plant with deep red flowers that grew on Amon Rudh.” - meanwhile, niphredil’s entry mentions that it grew in Doriath as well as Lorien. So, if seregon grew anywhere else, you’d think Tolkien would have mentioned it.
Then again, niphredil was specifically mentioned while the fellowship was in Lorien, so it could just be that Amon Rudh was the only place we (the reader) ever see seregon growing. Perhaps it grew throughout the region, but wasn’t mentioned until Amon Rudh (because of the striking image of the “bloody” hill-top.)
SOURCES: The Silmarillion