As Walter suffers from visions of his old lab assistant, Carla Warren (Jenni Blong) and images relating to his first passage to the parallel universe (as seen in "Peter"), the rest of the team work with Anil (Shaun Smyth) to triangulate the signal from the radio.

Peter (Joshua Jackson) and Olivia (Anna Torv) travel to the site in a nearby forest, and find the decades-old remains of Observers and members of the human resistance, including one with Sam Weiss's identification.

Walter is drawn by his hallucination of Carla to a spot in the lab where he had hidden a notebook containing many of his ideas, including the original design for the device to cross between universes.

Peter, Olivia, Walter, and Astrid (Jasika Nicole) travel to a boat dock across from the island site where the radio signal originates, briefly stalled by a Loyalist force.

"Black Blotter" was written by Kristin Cantrell, marking her first writing credit for the series; she previously served as a script coordinator.

[5] Jeff Jensen of Entertainment Weekly named "Black Blotter" the nineteenth best episode of the series, stating "Walter Bishop's most epic LSD trip ever — an attempt to recover lost memories of his master plan to defeat the Observers — allowed Fringe to creatively cut loose with some novel storytelling devices, none more inspired than a cartoon romp rendered in the style of Terry Gilliam's stop-motion Monty Python animation.