It was decided after the project began that if the dam were built at Black Canyon instead of Boulder Canyon, it would be able to capture more water. Also, geologically, Black Canyon had a more dense rock in its canyon walls. When the dam site was moved to Black Canyon, it was still called the Boulder Canyon Project. The dam got its name from the project which originated it, 'Boulder Dam'.

On September 17, 1930, Herbert Hoover's Secretary of the Interior Ray L. Wilbur, went to the site to dedicate the official start of the project. In his dedication speech, he announced that the dam would from that point on be officially known as Hoover Dam. This was a rather unpopular idea at the time.

In 1930, the Great Depression was getting worse and Hoover was either blamed for it or castigated for not doing  anything about it. Herbert Hoover wanting to be re-elected in 1932, felt that he needed to show that he was sensitive to the situation. By naming the dam after himself, he thought that he could draw attention to the fact that he was instrumental in starting the project. With over 5000 people to be employed on the project, Hoover thought that he could claim credit for trying to do something about the unemployment situation which was extreme at that time. Unfortunately for Hoover, it did not work out that way.

On May 8, 1933, Harold Ickes, Franklin Roosevelt's Secretary of the Interior, decided that the name of the dam should be 'Boulder Dam', its original name. The reason for this was no doubt political.
On April 30, 1947, the resolution renaming the dam back to Hoover Dam was passed by congress and signed by President Harry S Truman.

Hoover Dam is still, currently, the name of this structure. Note that the masthead image on the home page is taken looking towards Boulder Canyon.