Yea, we don't know who this Donald Blake is... More than likely it's the clone that IM made, that went and killed Goliath
You see, at first Donald Blake was a doctor, that turned into Thor. As it turns out, after a while, Stan Lee revealed that there never was a Donald Blake. Thor's father Odin, had sent Thor to Earth, and wiped his memory, to make him think that he was a human, Donald Blake. He did this as a test to see if Thor was virtuous enough, and deserved to carry the hammer, and the power of a god.
After many years, and many battles with his brother Loki, (the god of mischief,) as well as many other gods etc. Thor found out some major things about Ragnorak, (the twilight of the gods.) I won't go into major detail, (just look at wikipedia,) but basically it all went to s**t, and Thor aided in the to the destruction of Ragnorak, and many that were to call it home. Including Thor.
This was a few years ago now... One of the interesting things in Norse mythology, is that with the gods, there are stories of continuous death and rebirth... Although it's clear, that Thor ain't coming back that soon, and he is infact, very much dead. (Although JMS is supposedly slated to be doig a new Thor series sometime next year... Whether this will be the Thor that we've known in the past remains to be seen.
Turns out Thor was good enough, and his father revealed the truth. So although he became Thor full time, his time as Donald Blake grounded him withy morals, decency, and values.