Technically speaking I think an alterna phase has narrow black primary bands with alternating narrower broken bands or spots on a light to dark gray backround. Just as important as alternates is that there is no red anywhere on the snake.

A Blair's phase has wide red bands with black borders and a light to dark gray backround. Often the black borders have a secondary white border on the outside edge(between the black and the gray). There are no alternates.

An intermediate is anything in between. True Blair's phases are common. True alterna phases(ie no red at all) are actually relatively rare because most specimens have at least some red even if it's just in the nuchal. So technically speaking most of the specimens we refer to as alterna phase should probably be called intermediate.

In my everyday useage I hardly ever use the term "intermediate". If the primary bands are narrow I call it an alterna and if the primary bands are wide I call it a Blair's. For example if it's got narrow primaries with little red wedges and some alternates I call it an alterna. If it's got narrow primaries(with or without red) and no alternates I still call it an alterna but I usually say "alterna with no alternates". If it's got wide red and some alternates I call it a "Blair's with alternates". I will occasionally use the term intermediate when the primary bands are in between, neither narrow nor wide. So I guess for me it mostly depends on the width of the primaries and not so much on the presence/lack of red, or the presence/lack of alternates.

With the advent of the internet it seems to not matter as much what you call them because you can usually get or send a photo.
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