Blue Sky (also known as Big Blue, Blue Magic, or simply Blue) is the street name which has been coined for the notoriously potent and 99.1% chemically pure crystal methamphetamine manufactured by Walter White and Jesse Pinkman.
image of Walter and Jesse cooking
Walt and Jesse cooking Blue Sky in the early days

The name comes from the distinctive light blue coloration of the meth crystals, a result of the chemical formula Walt devised to get around sales restrictions on pseudoephedrine.

Walt and Jesse decide to move their business into new territory and turn to Saul for help. Saul uses his connections to arrange a meeting at a fast food restaurant with Gustavo Fring, a discreet, cautious, yet successful drug distributor. The blue meth spread across the Southwest due to the efforts of Fring, whose Los Pollos Hermanos franchise was used to distribute the meth to the surrounding states and establish a drug empire in the region.