The U.S. National Library of Medicine reports that the skin site of a boil sometimes itches as the boil develops. This is typically a warning sign that occurs prior to the manifestation of the boil.
According to WebMD, when a fully developed boil site begins to itch, it is usually a sign of infection or inflammation, which is known as folliculitis. Small pimples or red coloration often accompany the itching when folliculitis occurs. The skin around the site can also become tender or generally uncomfortable. The typical cause of both boils and folliculitis is the accumulation of staph bacteria beneath the skin.