My Bloody Valentine is most famous for elevating “shoegaze,” a dreamy style of guitar music named for the activity of manipulating the technology — literally, gazing at the pedals next to your shoes — required to conjure such a swirling sound. The band is also famous for disappearing: After releasing its second album, “Loveless,” in 1991, it was mostly inactive for nearly 20 years, as rumors of a follow-up swelled and dispersed. After reuniting to perform live, the band surprised everyone in 2013 with “m b v,” a new record that appeared online in the middle of the night with no advance notice, instantly crashing the band’s website as fans swarmed to download it.