TinkerPop is just the name of the project that contains Blueprints and the other projects you see on the TinkerPop home page. It is not on its own a project or library that you can use, rather just the name of the suite of tools for the graph technology space.
TinkerPop also refers to the collection of individuals (many of whom are representatives from different graph database vendors, such as Neo4j, Titan, OrientDB, Bitsy, etc.) One might think of TinkerPop as a graph technology guild or fellowship.
TinkerPop is not a specification, mandate or standard for writing graph applications, nor does it aspire to be. TinkerPop merely provides a set of interfaces that graph database vendors can implement (Blueprints) to get all the features of the rest of the TinkerPop stack (Pipes, Frames, Rexster, Furnace, and Gremlin) where each part of the stack provides a specific function in supporting graph-based application development.
UPDATE: As of version 3.x, TinkerPop is an Apache project - http://tinkerpop.apache.org/. Under 3.x, TinkerPop no longer has individual separate projects such as Blueprints, Pipes, etc. It is all a single project repository that has folded in all of those projects.
