I couldn’t find a straightforward explanation for knitting blackberry stitch — also known as the trinity stitch — in the round as when you knit a hat, so here goes:
The stitch is knit in multiples of four stitches. Knit in multiples of four rows until your desired length is reached.
Row 1: Purl all stitches.
Row 2: *k3tog, (p1, k1, p1) into the next stitch; rep from * to end.
Row 3: Purl all stitches.
Row 4: *(p1, k1, p1) into the next stitch, k3tog; rep from * to end.
I’m currently knitting a hat from the bottom up. CO 96, (k1,p1) for 30 rows, then blackberry stitch until I decrease. I’ll let you know how it goes. (EDIT: It was cute!)
One tip is to keep the 1st and 3rd rows — the purl rows — looser than you normally knit, because knitting three stitches together on the front-facing side will give you hand cramps. This is a remarkably satisfying knit, however, because the little bobs of yarn give you so much texture.