Metallica's 'Black Album' turns 20 this year and when looking back at the experience, producer Bob Rock had few good things to say about the experience. He even goes as far as blaming Lars Ulrich for the band's descent from the thrash metal sound they had on previous albums.

There are usually two schools of thought on Metallica's self titled 1991 record (more affectionately known as 'The Black Album'): The first being that it was Metallica's first commercial success and a great album, and the second being that it was the beginning of the end of the once potent thrash metal pioneers. Longtime Metallica producer Bob Rock did an interview with Music Radar and shed some light on how the shift in the band's direction came to be:

"They had broken through to one level, but they still weren't on mainstream radio. When they came to me, they were ready to make that leap to the big, big leagues. A lot of people think that I changed the band. I didn't. In their heads, they were already changed when I met them."

Rock further attributed the band's re-focused sound to drummer Lars Ulrich:

"I noticed that Lars played to James’s guitar, much like the way that Keith Moon played to Pete Townshend. That’s fine for some bands, but not every one. Lars wanted Metallica to groove more. AC/DC’s Back In Black was a big reference point as a rock record that grooved. I told him that in order to get that feel, he had to be the focal point musically. So on certain songs, the band played to Lars. They followed him. It made a real difference."

Metallica also made a major change in the lyrical content and the way James Hetfield wrote, Rock says that it was no accident:

"He wanted to go deeper with his writing. He wanted his songs to really matter. We talked about the great songwriters, like Dylan and Lennon and Bob Marley, and I think he saw that he could write for himself but still touch other people. It was a struggle for him, but he had a tremendous breakthrough as a writer."

Even though Rock said "It wasn’t a fun, easy record to make", he also went on to say of the experience:

"It’s probably the biggest accomplishment of my professional life, I think the band would say the same thing."

Maybe. Despite being the band's most commercially successful album, spawning 5 huge singles and selling 22 million copies; does selling more albums mean that it is the best music that you've made?