I've got it!
bLocator = new TLOBLocator(OrclSession, otBLOB);
OrclQuery->DeclareVariable("ora_blob", otBLOB);
OrclQuery->SetComplexVariable("ora_blob", bLocator);
OrclQuery->SQL->Text = "insert into _table values (..., EMPTY_BLOB(),...) returning _column into ra_blob";
OrclQuery->Execute()...