It is probably an art magazine like the book Art. Squidward can be seen reading it when SpongeBob wants to eat a Krabby Patty with Patrick in the Krusty Krab. It actually looks more like a comic book than a magazine, since it looks like it has only four pages. It could also be a magazine like Teen Ink, except for squids.
This magazine has a yellow cover. It says "Squid Ink," in blue/purple writing, and under that there is a blob of ink exploding from a pen.