^Was that Jeffrey Hunter?
It was towards the end of his career and his voice wasn't the best and he did the voice of Heathcliff the following year.
That doesn't make sense. It's not like a voice actor can only do one show at a time. There are plenty of voice actors who are working on half a dozen shows or more at the same time, because recording a voice part hardly takes any time at all. You can do a whole episode's worth in a few hours, tops (since you don't have to worry about wardrobe or makeup or cameras or lighting or any of that). Blanc was working on multiple Warner Bros. and Hanna-Barbera projects at the same time he worked on both seasons of Buck Rogers, so it's not like adding just one more gig would've been too much for him.
Also, Blanc was still working steadily until his death in 1989, so it's totally untrue that his voice was failing him in 1980-81. According to a speech therapist who studied his throat and larynx late in life, Blanc had the most incredibly robust vocal apparatus the man had ever seen and he didn't expect it would ever give out.
Define steady, Heathcliff was his last series voice work other than that he did commericals and many of those were live action, his son Noel had taken over many of voice work by then. I still haven't seen your explaination though.