Actually, it happened in Amazing Adventures #11 from 1971 (and later reprinted in Avengers #137), here's the scoop (and probably more than you wanted to know):
First about the book itself: Back in the 1960's Marvel had several titles that had two different half-issue stories in them. Tales of Suspense had Ironman & Captain America stories and Tales to Astonish had Hulk & Submariner Stories. In 1968 all four heros got their own books. In 1970 they decided to try that split comic format again, launching Astonishing Tales with Ka-Zar and Dr. Doom stories, and Amazing Adventures with Black Widow & Inhumans stories. Neither sold well, and they abandoned the split format with issue #8 of both books, continuing on with a few full-issue stories of Ka-Zar and Inhumans to complete ongoing plots. From that point on they used the two books to launch new solo heroes. Astonishing Tales being the springboard for both Man-Thing and Deathlok, while Amazing Adventures lauched the new and improved Beast and Killraven.
The new and improved Beast: The beast's run on this book lasted for seven issues (#11-#17). This took place during the five-year span between X-Men #66, the final issue of the original X-Men, and X-Men #94, which began the new X-Men (#67 to 93 just being reprints of earlier stories). During that span the original X-Men only had a handful of cameo appearances in other books, so were available.
The storyline has the Beast saying goodbye to the X-Men and going to work for the Brand Corporation. While there he tries to isolate the gene that causes mutation. There are villains involved, from a rival corporation, and he winds of ingesting a vial of the gene which causes him to mutate further. He is actually grey in the first issue, but they then changed it to blue. He wore Hank McCoy rubber masks after than to hide his transformation.
They decided to be "cute" and also toss Patsy Walker into the storyline, who Marvel and its predecessor company Atlas Comics had featured in an Archie-like girls love comic book series that had over 250 issues between 3 titles from 1945 and 1967. At the end of that series the teenage girl had married her true love, a soldier named Buzz Baxter, who was now head of security at Brand. Patsy discovers Hank McCoy's secret identity and becomes his partner. The series ended before the creative staff was done with it, so they just transfered the plotline over to their next book, The Avengers. Patsy went on to become the Hellcat.