Foreign key: in which table?
In your blood_types table,

CREATE TABLE blood_types
(
 blo_id serial NOT NULL,
 blo_name character varying(10),
 CONSTRAINT blood_types_pkey PRIMARY KEY (blo_id)
)


You correctly have blo_id as a PRIMARY KEY.

And then in the persons table, have the per_blood_type integer as a FOREIGN KEY referencing blo_id in table blood_types (as you appear to have - can't check exact syntax - don't have running PostgreSQL system at the minute). The way you have have it appears to be the correct way - blood type depends on the person and only the person - not the other way round.