Mastodon guitarist Bill Kelliher was just 20 years old when Metallica's "Black Album" first hit record store shelves. He was already a big fan of the band, but he recalls being indifferent to the streamlined heavy metal sound the thrash legends debuted with the record.

At first, anyway.

Listeners familiar with Metallica history are well aware of the stylistic shift the group employed on the self-titled "Black Album," their first to wholly eschew the breakneck metal of their early albums in an effort to open up their appeal. Of course, the move was a massive success, and the album is still reaping the benefits of its explosive mainstream popularity today.

But back when it first emerged, some who already rocked Metallica found the progression a bit jarring.

Recently asked for his favorite guitar moments on the "Black Album," Kelliher, Mastodon's rhythm guitarist and backing vocalist, looked back on the pivotal LP.

"The thing about the 'Black Album' is that there are so many fucking awesome riffs on it," the rocker told Guitar World on Monday.

"When I first heard it, I was a bit, not disappointed, but because of the cleaner singing I was kinda like, 'Whatever!'" he explained. "But later I went back to listen to it and was like, 'Holy fuck!' That record is so good. It's an absolute crusher. I could name any song or any riff as a defining moment."

Kelliher added, "The 'Sad But True' riff is perfect, for example. I actually learned it on guitar pretty recently because it sounds so simple yet sounds so big with the palm muting. It's a real chugga-chugga record, every song has lots of that going on… which I love."

On top of that, the musician marveled, Metallica "went from writing 10-minute opuses to this. If you listen to …And Justice For All, there's so much technical wizardry and so many tempo changes."

With the "Black Album," Metallica got "straight to the point," the Mastodon member continued. "They'd get to the chorus quicker, which is more of a rock and roll or pop kind of approach, going from verse to chorus to verse to chorus to solo to chorus. … Hetfield really went out on a limb when he started actually singing. I know he was inspired by [rock crooner] Chris Isaak a lot on this record."

The 1991 album from Metallica, known colloquially to the band and fans by the color of its album art, was reissued in a remastered edition last month. In the lead-up to the re-release, Metallica teased bonus album cuts in a rare version of "Sad But True" and other tunes.

Mastodon's Hushed and Grim arrives on Oct. 29. It's preceded by "Pushing the Tides" and "Teardrinker." The band will tour the U.S. with Opeth next month.

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