Fan blade failure isn't unknown. Three widely reported incidents:
1989 January 8,
British Midland Airways 92.
A fan blade fractured due to aerodynamic flutter.
2016 August 27,
Southwest Airlines 3472.
Part of a fan blade broke off, after the engine ingested debris.
2018 April 17,
Southwest Airlines 1380.
A fan blade separated at the root.
Page 4 of a 1997 FAA advisory circular quotes an SAE report of
676 uncontained engine failures from 1969 to 1997, but doesn't discriminate between blade failure and other failures.
Page 19 of a 1991 SAE Report on aircraft engine containment may have the closest answer: high bypass turbofans had 1.12 events per million hours, and 0.56 when excluding fan blade events; subtracting,
we get 0.56 fan blade failures per million hours that resulted in uncontained engine failure, for commercial transport during 1976-1983. Blade failures that did not cause uncontainment aren't counted here, so the true number should be somewhat higher. This hundred-page report goes into eye-watering detail.