Is it possible to export a specific area to blender using the Cesium box excluder?
No, sorry, the excluder is for excluding parts of a model from rendering, not for exporting to other tools.
However, you can use Cesium Clips to export part of a tileset to glTF, which can be imported into Blender:
Note that not all tilesets can be clipped, because it is against the terms of use of some providers. Google Photorealistic 3D Tiles falls into this category, unfortunately.