Color is one of the things that can help or hinder your ability to date a piece of Blenko glass. Some colors were made only for a year or two, others have been made almost every year. The Blenko Four website has a whole section devoted to colors, that helps differentiate the shading and the time periods they were made: http://www.blenkocollectors.com/blenkocolors.htm Check the bottom of the page for photos of color groupings. I highly recommend this to new collectors wanting to learn about Blenko glass, especially regarding colors that to an untrained eye can appear to be the same.
From my own collection: On the left is an amethyst ivy vase. I am not positive this is Blenko, but the pontil feels right, and the color is the same as other pieces I can positively identify as Blenko glass. The decanter is a 6310 in Rosé. Rosé was in the catalog in 1963 and 1964 only, so I know this piece was made one of those two years. The dented vase is a Hank Adams design, #894 in Dusty Rose. I know this was made between 1989, when the vase was designed, and 1990, which is the last time the color Dusty Rose was made, and because the design no longer appears in the 1991 catalog. In 1991, Dusty Rose was replaced by the color Orchid.