https://sandbox.babylonjs.com/
this is what shows on the screen
but it should be cube with edges
can help
Well that link to the Sandbox does not help - just gives the Sandbox for me to load a file and textures from my HD. Maybe try loading your export from Blender into the Playground?
cheers. gryff 
I think your problem is related to shading.
In order to obtain a smoother rendering, Babylon uses normal shading as a base. On the contrary, flat shading allows you to display each individual faces of your mesh.
https://doc.babylonjs.com/resources/normals#normals-and-flat-shaded-meshes
@1111 you can also add an edge split modifier to geometry in Blender, or a small bevel to edges you want to remain sharp, before exporting to Babylon.
check this link too https://doc.babylonjs.com/resources/blender_tips#smooth-shading (this is for .babylon. For .gltf, use Blender mesh autosmooth + sharp edges)