Black Balloon Publishing is an imprint of Catapult Books. They have online and print distribution. Their books tend to be well reviewed and many have been blurbed by famous established authors. They publish literary fiction, non-fiction, and memoirs. They try to focus on publishing things that are odd or quirky. It would be appropriate to use the term “hipster” to describe most of their books.
They also run a very popular blog in the book community called the Airship that draws a fair amount of readers in.
Black Balloon Publishing only has ten books out right now. Because of that small number of books it is easy to see they are rejecting a fair number of manuscripts. The books that they have published include an illustrated novel, a novel in translation, a collection of short fiction, and an anthology of writers writing about their favorite bars. It is good to look over what they have previously published to see if your work would fit in there.
To submit your work to them you use submittable, the submission manager used mostly by literary journals and a few small presses. There submittable has a separate category for the submission of illustrated novels, so clearly that is a sub-genre they are particularly interested in. You should include a cover letter with your manuscript submissions. They have no specific guidelines for what the cover letter should contain.
To learn more about Black Balloon Publishing you can visit their website here. They usually only read manuscripts twice a year. This year they plan to be open in August. They also host contests (but fees are involved).