MCU HISTORY
During the 1930's, a Dr. Abraham Erskine works in a German university doing research into the serum. However in 1935, the Jewish doctor left his work in an attempt to escape Germany to avoid the racist Nazi laws but HYDRA (Nazi special weapons division) leader Johann Schmidt, who took an interest in Erskine's work, intercepts the scientist on the German/Swiss border on September 14th.
5 years later in the Beverian Alps, Schmidt takes part in an test involving Erskine and his Super Soldier Serum which turns Schmitd into what would become known as the Red Skull. Erskine is rescued by Agent Thirteen (Peggy Carter) and brought into the US Secret Science Reserve as part of the Project Rebirth program along with Howard Stark to produce America's own Super-Soldier.
After a long search for the perfect subject, the frail Steve Rogers is found and transformed into the perfect human specimen; becoming Captain America. However during this experiment, Erskine is killed by a HYDRA agent and the knowledge of the serum is lost with him.

This blue coloured serum is seen again on May 20th 2010 after being long locked away as part of the Weapons Plus program under the name of “Dr. Reinstein” (most likely the codename of Erskine - a similar retcon was used in the comics to cover up the name discrepancy) when General Ross uses a low dosage on a soldier named Emil Blonski.
The Serum and its affects on its subjects
The serum effect on the body is to increasing the molecular density of cellular fibers (skin, muscle) through synthetic proteins. However, the serum not only enhances the the body and mind but also brings out and enhances what the person is. These personality traits are brought to the surface and manifest physically on the subject.
Steve Rogers
Erskine's insistence to find a "good man" is an understatement, in Steve Rogers he found someone with unquestionable moral value who only wanted to be the best he could be and his physical transformation is a reflection of that.
Emil Blonski
Given low dosages of the serum, Blonsky's extra physical manifestations (protruding skeletal structure) takes a while to materialize. Despite the Hulk blood kicking the serum into overdrive, giving Blonsky a giant sized version of himself with a green tinge, i believe if he had taken a full dose of serum his appearance would have been akin to that of X-Men character Marrow, with the external bone structure being a reflection of the tough monstrous fighter that he shows himself to be.
Johann Schmidt
Most likely the first to take the serum, Schmidt's external transformation is that of a living Red Skull. What sort of internal trait is being personified by this? frankly, i dont know for sure. The best i can come up with is that Schmidt has always wanted to be seen as removed from the mere ordinary people around him, he is above them and they should be scared by the pure sight of him and his appearance is a reflection of that.
My Thoughts
At first i wasnt sure on this new twist for the serum (that it bring out whats inside its subject) but after some though i have warmed up to the idea, the serum amplifies every other aspect of the mind and body so it is a natural progression to say that it also amplifies there own personal traits as well. This also opens the doors to what the serum can do in any other future use.
Back in 2008 after watching The Incredible Hulk, i assumed that this serum used by Ross was an unstable copy but now with the new information im thinking the it was in fact some of the last remaining original serum (several unused vials are shown). Also by the talk given to Blonsky before taking the serum, im guessing that he was not the first soldier since World War II to receive the serum. Possibilities of this are endless but the 50's Captain America springs to mind as Ross stresses the idea that the serum could make the subject mentally unstable.
I haven't come across any solid info on why they included the serum in TIH or if they intended for its use to mimic the twist they have put on it for its appearance in Captain America: First Avenger, but knowing Marvel there is more reason behind these things than any of us can piece together.
What are your thoughts on serum and the film take on it, who else do you think has had the serum since World War II, and how would you like to see it used in the future. Leave your comments below.
Make Mine Marvel!
Sources
The Incredible Hulk (2008)
Captain America: First Avenger - Trailers/TV Spots (2011)
Captain America: First Vengance #1-4 (2011)
Film Buff (Marvel Cinematic Timeline)