Blue Grotto are a shaped rocks in the northern part of the island of Zakynthos. Of course the cave in general is practically around the perimeter of the island, especially in places where the cliffs fall into the sea simple. However, the most famous are the Blue Caves and Keri Caverns.
To Grottos you can get only from the sea, so practically every village on Zakynthos (and local guides at each, even the smallest beach) offer a trip to the Blue Grotto. Often these trips are related to cruise to the Gulf of Navagio (Wreck Bay).
Like the trip to Navagio, it is worth to go from the small ports on the northern part of the island (Agios Nikolaos, Skinari etc) due to the short distance to Grottos, sail small boats, which can literally enter the Grotto. Often in smaller caves these boats stop to be able to bathe inside the cave. It is quite an interesting experience - the water seems that it isn't like the one outside:-)