As you might have noticed the Microsoft Expression Products[1] got their final names and a new addition. In true adherence to other naming schemes[2][3] they decided to give Interaction Designer a name that is 'easy' to recognize. I remember it by remembering that it's the one with the non descriptive name... Let me think.... ah! Blend (was it?)
From the FAQ[4]
Q. Are these the final names of the Expression family of products?
A. Yes. Expression Web was previously called Expression "Web Designer", Expression Blend was previously called Expression "Interactive Designer", and Expression Design was referred to as Expression "Graphic Designer". Expression Media is a new addition to the Microsoft Expression family of products introduced to the public on December 4th 2006. These new names are the official and final product names.
[1] Expression Products
[2] Oh dear, Avalon and Indigo got there real names
[3] Again- new names
[4] Expression - FAQ