Most silicone sealants can be used to seal a sink but a specific mould resistant sealant such as Dowsil 785+ will work best.
When sealing a sink or bath most silicone sealants will do the job. However, some sealant are produced with mould resistant chemicals that help to prevent the sealant from going black. Dowsil specialises in producing high-quality sealants for a variety of applications with Dowsil 785+ being specifically designed to reduce the build-up of mould. However, if you are looking to seal a sink in a kitchen area then Dowsil 786 may be more suitable as this sealant is safe for food contact but is available in fewer colour options.
TIP: Black mould on silicone is caused by the chemicals in the soap whilst washing. To prevent black mould forming wash the silicone with clean water after every shower or bath. This may not prevent the build-up all together but will increase the time until the silicone starts to discolour.