1. The Black Keys
Before The Black Keys were making hit songs like Gold on the Ceiling, Lonely Boy, Little Black Submarines, Gotta Get Away, etc. they were a lo-fi gritty, soulful, thick edged blues band. In fact, they had this sound for their first 4 or 5 albums before they started transitioning on records like Attack & Release and Brothers into the modern pop rock they are known for. While understandably Dan Auerbach and Patrick Carney need to evolve as musicians, this move alienated a great deal of their long time fans who followed them for the first 7 years of their career.
2. Fleetwood Mac
Fleetwood Mac is another example of a blues band that turned into pop rock. Before well known members Stevie Nicks and Lindsey Buckingham joined the band in 1975 they had been known for their straight up approach to blues, but with elements of hard rock and prog rock layering in later on in the early line-ups of the band. Founding frontman Peter Green was the primary songwriter and was a bluesman to the core, though as previously stated he did explore a lot musically later on as it was the late 1960's/early 1970's. However, in the mid-70's the band was about to split up after going through multiple replacements for Green when he quit in the early 70's. The band then hired Nicks and Buckingham and went the more pop direction they are known for today.
3. Def Leppard
Believe it or not, Def Leppard wasn't always a highly polished catchy hook delivering machine. There was a time where they played straight up heavy metal and could keep up with the best of the big boys in the early 1980's. Their first two albums On Through the Night and High n' Dry proved that they knew how to rock and had legitimate skills. As time went on however, they started favoring a more commercial style that would help them sell records. That's when the big hair, ear pleasing vocal harmonies, MTV videos, and earworm melodies began finding their way into the mix. Unfortunately, these albums have gone mostly forgotten and you have to go digging for them to know of their existence and find yourself a copy.
4. Avenged Sevenfold
When this highly regarded metal quintet began in the late 1990's, they were a metalcore band through and through. M. Shadows only rarely did much of the way of melodic singing in those days. Their first album Sounding the Seventh Trumpet is the prime example of this. However, by the time they got to their second album Waking the Fallen, they were already beginning to transition to the more commercial/radio friendly style they do today. The album was half metalcore screaming and half tradition metal singing. To be honest, that was actually their best sound in my opinion. Unfortunately, the band has stated recently that they have no desire to do any more music in that style.