где то стырил, не помню где уже

FUNCTION get_blob_from_base64_string(p_clob CLOB) RETURN BLOB IS
l_chunk BLOB; --Chunks of decoded blob that'll be appended
l_result BLOB; --Final blob result to be returned
l_rawout RAW(32767); --Decoded raw data from first pass decode
l_rawin RAW(32767); --Encoded raw data chunk
l_amt NUMBER DEFAULT 7700;
--Default length of data to decode
l_offset NUMBER DEFAULT 1;
--Default Offset of data to decode
l_tempvarchar VARCHAR2(32767);
BEGIN
DBMS_LOB.createtemporary(l_result, FALSE, DBMS_LOB.CALL);
DBMS_LOB.createtemporary(l_chunk, FALSE, DBMS_LOB.CALL);
LOOP
DBMS_LOB.READ(p_clob, l_amt, l_offset, l_tempvarchar);
l_offset := l_amt + l_offset;
l_rawin := UTL_RAW.cast_to_raw(l_tempvarchar);
l_rawout := UTL_ENCODE.base64_decode(l_rawin);
l_chunk := to_blob(l_rawout);
DBMS_LOB.append(l_result, l_chunk);
END LOOP;
return l_result;
EXCEPTION
WHEN NO_DATA_FOUND THEN
return NULL;
END;