I had a similar experience in the 00's, I blind purchased the used CD from a local store called Replay Records. Definitely a mind-expanding experience cos I hadn't heard much prog/tech thrash yet, certainly not a full album of the style.
In the late 50's and early 60's Dio was singing doo-wop, fast forward 30+ years and he's singing this heavy as fuck song. What a life that guy had man.