DEFINITION Blue refers to water, and it essentially means that water is being placed (or held) on the roof on purpose to store water (typically rainfall) for reuse such as irrigation or cooling water makeup, or recreational opportunities.
AKARoof pond, Roof Spray
PERFORMANCE CRITERIAThe reason's for creating a blue roof vary. In some climates this is a water detention strategy, in others it is a means of keeping water perched above the building for cooling reasons. Still other authors point to water (plus some aggregate) as a way of "seeding" a poor man's green roof. Over time, vegetation will settle upon the roof, take root and provide all of the benefits of a
green roof.
ILLUSTRATION

Images courtesy
Urban OmnibusCASE STUDIESlinks to separate pages documenting specific places or projects where this element is on display
ADDITIONAL RESOURCESLandscape + Urbanism provides a breakdown of the various eco-roof typologies
here.Another good breakdown is at
Urban Omnibus.
An blueroof project:
The "Urbeach" urban rooftop renewable energy from beachlike space.
PlaNYC Stormwater Plan talks about Blue Roofs as LID hereSome more general info here