Oracle blob to clob

 function f_blob_to_clob(blob_in in blob) return clob as
    v_clob    clob;
    v_varchar varchar(32767);
    v_start   pls_integer := 1;
    v_buffer  pls_integer := 32767;
  begin
    dbms_lob.createtemporary(v_clob, true);

    for i in 1 .. ceil(dbms_lob.getlength(blob_in) / v_buffer) loop

      v_varchar := utl_raw.cast_to_varchar2(dbms_lob.substr(blob_in,
                                                            v_buffer,
                                                            v_start));

      dbms_lob.writeappend(v_clob, length(v_varchar), v_varchar);

      v_start := v_start + v_buffer;
    end loop;

    return v_clob;
  end f_blob_to_clob;

or use apex wwv_flow_utilities.blob_to_clob