Blur is a Phaser and is a member of the Black Swan's Collective, along with Mr. Forkle, Squall, Wraith, and Granite. To disguise himself, he breaks his body down and only reforms partially, leaving the only parts of his body visible as splotches of color and blurred lines. He was first introduced in Neverseen and makes appearances in Lodestar and Nightfall.
When Sophie and Keefe go with Lady Cadence to Ravagog in Nightfall, Blur comes with them and is frequently splashed by water. Part of his disguise fades and Sophie notices that he had full lips and round cheeks. She also thinks he looks somewhat familiar.
Fan Theories - Blur's Identity
Please note: Blur's identity is currently unknown and these theories could be incorrect.
The Theory - It was mentioned in the books that Jensi's older brother is a Phaser, but nothing else is mentioned about him. Shannon does not just drop facts randomly so this leads many readers to believe that Blur is Jensi's older brother. In fact, Blur shares several similarities in appearance with Jensi himself, and Jensi and his brother probably have some similar features. In Nightfall, on their trip to Ravagog, Sophie sees some water wash over Blur, causing his outline to be revealed for a split second. Sophie comments on how he is not only skinnier and shorter than Sophie thought but his round cheeks and full lips looked familiar.