Blackmore, who became the first ever female winner of the Grand National earlier this year, was thrown from favourite Merry Poppins during the 3.25 race at Killarney.
Having guided Baltinglass Hill to a win just half an hour earlier, Blackmore was odd-ons to record back-to-back wins, but a stumble over a hurdle saw mount and jockey land heavily.
A statement from Dr Richard Downey at the racetrack read: "Rachael Blackmore is being taken to Tralee Hospital for further assessment on an upper leg injury following a fall in Race 4 at Killarney.
"She is fully conscious."
Racing TV’s Kevin Ryan said: "Rachael has been moved into the ambulance and taken to hospital with an upper leg injury.
"She is fully conscious and can move everything. She had to be stabilised by the on-course medics before she could be moved."