Bluo-Gal is used as an alternative to X-Gal, and produces a darker blue color. Bluo-Gal is hydrolyzed by β-galactosidase to produce a blue precipitate. This precipitate is insoluble in alcohol and xylenes, thus making it suitable for immunoblotting and immunocytochemical assays. Bacterial colonies containing active β-galactosidase produce blue colonies when grown on plates containing Bluo-Gal.