I will continue this discussion on the Board, but it may be better to exchange e-mails unless other Board readers are interested.
There are several possibilities for the origins of your "Isaac" line. Since this thread was titled "Blanchards of Rhode Island", can I assume that Isaac's line has a connection to RI?
I manage the BLANCHARD List at RootsWeb and offer the following suggestions. Have you searched the BLANCHARD List Archives at http://archiver.rootsweb.ancestry.com/cgi-bin
Some key search words relative to Isaac (names, locations, dates) may return some useful information in the List messages that go back to 1996.
Many of the messages refer to descendants of the immigrants Thomas (1639) known to have immigrated to Boston from England and Joseph who arrived in Boston in 1637 and is only presumed to have come from England. Our DNA Project has proved that these two lines are not related though this was thought to be true for many years.
Numerous other lines have connections to Canada and many BLANCHARDs came to America across the borders of northern New England, New York and states farther west. In addition, several lines have connections to the Acadians.
To be more specific, since Jefferson and Lewis Cos, are in the NW area of NY, Isaac's ancestors may have come from Canada either directly or from some other earlier location such as New England.
If you have proof that Isaac has ties to RI, I may be able to help you connect him to one of the early lines that members of the BLANCHARD List have confirmed.