It is a traditional granny square afghan, I've made one myself a few years back (before I even saw the Dark Shadows episodes in color on Hulu, as a kid the first time around, I didn't have a color TV, so I never noticed it back then)
The pattern is very simple: Chain 4, join with slip stitch to form a ring. In the ring, double crochet three double crochet stitches, chain 1, three double crochet, chain 1, until you have 4 groups of three double crochets with a chain between them in all. Join with a slip stitch at the beginning. Change colors.
Round 2: Three double crochet, chain one, then three double crochet in each chain one space of previous round. chain one, continue the same in each chain one space, join in beginning of round with slip stitch (this makes the four corners). Change colors.
Round 3: Three double crochet in chain one space to the corner. In the corner, make three double crochets, chain one in corner, three double crochetss Next chain one space, three double crochets, then chain one. Continue until the end of round, join with slip stitch. Change colors.
Round 4:Same as round three.
Round 5: Same as above, only use black or another dark color.
Most of the time, there are four color rounds and one with a dark color as a border. You can use any colors you wish, I've made one that looks exactly like the one on TV and another with solid colors in the first three (or you could do four) rounds with a white border. The other afghan they show seems to be red and yellow and is more like that. There are any number of color combinations possible, and the size of the squares can vary. The squares are stitched together with a yarn needle, or some people crochet them together. They are a good way to use up leftover yarn from other projects, years ago, this is probably how they came about, like making quilts. People couldn't afford to waste anything, textiles were expensive. Hope this helps!
I just think it was an overused prop-somebody's mom or grandmother probably made it, it looks antique, and since very few people had color TV's at the time, nobody would really notice if they used it a lot. It was a low budget show, after all. You sometimes see the same lamps and furniture if you look closely enough. And they probably never thought that the tapes would survive and people would still be watching 50 years later!
This is great!! I was just doing some research on where the Collinsport Afghan appeared, and now I see that someone has already done that for me! I know a lot of people out there who will be happy that the granny square pattern is actually posted here! I keep telling people to look up the directions on YouTube!!