I did some searching to try to find a crochet join that had more of a visual presence than a simple slip stitch join but that would not add to the dimensions of the blanket and I didn’t find anything that I liked, so I winged it. What I did was still essentially a slip stitch join but instead of working the slip stitches through the corresponding stitches of both blocks at once I alternated, working a slip stitch in the back loop of a stitch on block A and then a slip stitch in the back loop of the corresponding stitch on block B. I joined all of the horizontal rows first and then joined the vertical columns. At each intersection between 4 blocks I worked one chain stitch when crossing over the existing perpendicular join. The outside border is single crochet.

The squares made with the Caron yarn came out slightly larger than then ones made with Red Heart, so I steam blocked the Caron squares first without stretching them at all and then blocked the RHSS squares stretched slightly (about 1/2”) to match the size.