You might try this web site. It give a very detailed explanation of the history of Immaculate Conception (and is Roman Catholic friendly). The concept of Mary's sinlessness evolved over centuries and never was original teaching by Christ nor His disciples...
http://www.udayton.edu/mary/resources/kimmac.html
Correct, the Church meditates on the deposit of faith (scripture and sacred tradition) and over time arrives at a fuller understanding of it. In this way Christianity is a living faith
This is not new revelation it is a new understanding of revelation.