Blue carbon is the atmospheric carbon which in some form or another is stored in the ocean. This storage can be beneficial or have negative impacts. Blue Carbon Sinks for instance are natural resources that efficiently store carbon and lock it away - a great example being photosynthetic coastal ecosystems such as kelp forests; mangroves and seagrasses.

There is a 'darker side' to oceanic carbon storage however. The uptake of atmospheric carbon into th ocean is adding more hydrogen molecules and therefore increasing the acidity of the ocean, feeding ocean acidification.

This is a vey basic answer, and a late reply (sorry!) but if you have any questions I'll do my best to help.