Darkole saves energy because the search screen is predominantly black, since "a given monitor requires more power to display a white (or light) screen than a black (or dark) screen." Roberson et al, 2002
The concept for a Google black search was proposed in January 2007 with a blog post titled Black Google Would Save 750 Megawatt-hours a Year. The post proposed the theory that a black version of the Google search engine would save a fair bit of energy and help the enviroment due to the popularity of the search engine.
The vast majority of monitor screen sales are now made up of LED backlit LCD's which do not light up the parts of the screen that are black, unlike older CCFL LCD monitors that worked by wastefully backlighting the entire screen. Thus Darkole can save consumers energy by not lighting up vasts parts of monitor screen. This energy saving can result in longer battery life on mobile and tablet devices.
Darkole may also reduce or eliminate the effects of visual stress for some individuals. Which may improve a number of aspects for users, including reading speed, attention span, comprehension and fluency.