My organization's Ethics committee has actually addressed this. During employment interviews, managers are supposed to go over all the 'stuff' that is required by their patient population - most of them have a document that lists everything. It includes various clinical interventions, work hours, shifts, etc. They ask "is there anything on this list that you would be unable to do?"
This raises the issue without delving into "forbidden" territory of religious beliefs. If any issues are addressed, it is up to the manager's discretion to decide whether or not to hire the individual that cannot administer blood, work on Saturdays, wear Level IV isolation garb, etc. If blood administration is a frequent event, managers are highly unlikely to hire an individual who can't/won't do it. They just have to be consistent & apply the same criteria to every applicant in order to avoid any charges of discrimination / adverse selection.
This process works for us.