Anery (Anerythristic) itself refers to a lack of a pigment called Erythrin. Erythrin is responsable for the lighter red/pink/salmon shades that Boas (aswell as other animals) produce. So the difference between a normal/classic Boa and an Anery boa is the Anery has a lack of light coloured pigment so should look overall darker and more contrasting.

Although saying that there are quite a few common Boas that are pretty dark and show little in the way of light coloured pigment. The difference then offcourse is the Anery is able to pass along an Anery allele to it's offspring which depending on the second parent might help create other mutations