Meth does not turn blue if above 90% purity... the purer crystal is the clearer the shards are. Unpure meth is cloudy and you cannot see-trough

"Blue meth is essentially the same as regular methamphetamine. The blue coloring, however, is like a marker to let drug dealers and consumers know which group produced it."

"The substance was field tested but since the methamphetamine was already blue, and is supposed to turn blue when mixed with the reagent, they were not certain that it was positive for meth. Testing performed later on the substance revealed that the product was blue chalk mixed with a cut of some other material"

"It is still unknown why the meth is blue or what the color is supposed to indicate — high quality, a specific source, or possibly a method of production. In one report it was mentioned that because the substance was blue, it was difficult to field test, since it was supposed to turn blue when mixed with the reagent. It was later reported to be blue chalk.
There has also been speculation that the meth chemist was imitating the television series “Breaking Bad” (2008), a drama about a high school chemistry teacher who turns to cooking meth to pay for his cancer
treatment. His meth was blue and was supposed to be of high quality."