Wikipedia sums it up. The Donald Blake "entity" was transferred into Stormbringer for Beta Ray Bill to change into a normal version of his race. Thor's father Odin decides his son needed to be taught humility and consequently incarnates Thor (without memories of godhood) on Earth as a mortal, partially disabled human medical student, Donald Blake.[55] After becoming a doctor, Blake witnesses the arrival of an alien scouting party while he is on vacation in Norway. Blake flees from the aliens into a cave. After discovering Thor's hammer Mjolnir (disguised as a walking stick) and striking it against a rock, he transforms into the thunder god.[56] Later, in Thor #159, Blake is revealed to have always been Thor, Odin's enchantment having caused him to forget his history as The Thunder God and believe himself mortal.[57]
In recent years I believe Don Blake has returned and I am sure is actually something different all together in a modern Marvel "Let me explain how Stan and Jack were wrong" way.