GridMaps are much closer to what you want to implement.
Edit: Since you really want to use MultiMesh.
Mesh and MultiMesh are Resources, containing information about what to display. MeshInstance and MultiMeshInstance are Spatials which can display them.
Basic code to create a MultiMesh can be found here. Once created, simply put it into a MultiMeshInstance in the tree to display it.
Since you will probably have quite a few blocks, you may want to use MeshLibrary anyway, even without any GridMap (for convenient storage). If the default way of creating one is insuficient for you, see this answer to create one by code.