It is named something utterly prosaic, Horizontal Chain. It should be called Cool Jazz-Like Square of the 1950s. This color combination works well too. It has a sour-candy look that suits the design. It is a sky/azure blue with dark purple.
I had to wait more than a month after starting the afghan for this dark purple Cascade 220 Superwash yarn. So I couldn't make squares with purple for a long time. I ordered from the online store of Roxy Yarns in Brooklyn. I understood from their website that they were in the process of moving, so maybe I can cut them some slack. But I waited and waited and the yarn didn't come. There was no communication regarding a delay.
I emailed and called and finally got the response that they were out of stock and would be posting the parcel in about a week. Of course the deadline came and went, and no yarn. When they finally sent the yarn, they did refund the shipping charge as a courtesy. That was nice of them, but I would rather pay and get good service. In contrast, Jimmy Beans, where I got some of the other colors, gave lightning fast service and great communication.
I also made this mosaic square. It is of the azure with a deep blue called Blue Velvet:
I realize that one thing I love about this afghan is that it forces you to make a large swatch of many different patterns. This gives you an intimate feeling for each stitch pattern and a good view of how it looks and behaves. For example, Ruth of Ruthless Knitting recently made a little carseat baby blanket using the Rose pattern from the first Learn to Knit Afghan section (Knit and Purl designs). I had just been thinking that it would make a cute baby dress in bright colors.
Each of the squares you see here is sitting inside the little blocking device Barbara Walker advises you to make. It is a traced outline of the prototype square (Garter Stripe) stitched over with embroidery thread, made upon the ironing board cover. Here is a picture of it.
Not a very pretty picture, but it works well. Each square is steamed to shape and pinned. It dries in no time and becomes the uniform size and shape. Some shrink a little when removed from the pins, but not too much to stretch back when it's time to sew them together.