We have had "Black African" granite installed in several houses over the years, including in Thailand and have NEVER sealed any of it.
The wife cuts Limes and Lemons on the granite every day and it never stains.
While some stone types do need a sealer to stop staining (marble, Limestone ETC) high quality, dark, polished granites do not require it.
Many Composite worktops are susceptible to staining, mainly due to their porous nature and will benefit from a sealant being applied.
Back in the days when I worked, one of the companies in the group I worked for, built a shed load of buildings in London and around the World.
The bottom (street level) courses of the buildings were faced with polished
granite, simply because it repelled dirt and grime thrown up by footfall and traffic ETC. None of it was ever sealed.
Some good info on this site.http://www.countertopspecialty.com/granite-sealer.html