I waited to reply, hoping someone would come up with something more specific, but here goes:
Land records, wills, older family histories in book form, county and local histories of early settlers, hoping to find someone who has the family Bible, and hoping to find someone who has already done the research and is willing to share it.
Their first census was 1820, correct? 1820 to 1840 only listed heads of households and composition by age and sex. Can you make any sense out of them working backward? For example, can you find Blake households that hold males of the age that your great grandfather would have been in 1840, 1830, etc? Can you find the John Blake named on that petition on those censuses? Were there other Blakes in the area? Did that John live long enough to be on the 1850 census with a place of birth?
Did your great-grandfather marry someone from the area? Does her surname appear on any early lists or petitions? Did everyone in Montgomery County come from the same place before that?
Get to know the history of the area and specific resources available, genealogy societies, rootsweb or genweb pages, etc. Google everything - you might turn up someone's small website with what you are looking for.