This site:
http://www.decks.com/how-to/57...
says this about using them to support a deck, but it is not talking about a steep slope.
"They must ... and be a minimum of 12-inches below grade, or below the local frost depth. They cannot, however, be simply placed at grade level."
It sounds like you are right about burying them.
This has a detailed step-by-step plan and photos for putting a shed on what looks like a gradual slope:
http://www.oneprojectcloser.co...
However, it uses concrete poured into holes, not pier blocks.
