The texture flags for NIF exports from Blender for Fallout 3 include the SF_SHADOW_MAP and SF_SHADOW_FRUSTUM flags in the BSShaderFlags property. These flags seem to prevent display of the textures in the G.E.C.K. and in-game, causing invisible objects. This likely also applies to Oblivion but I don't have a way to verify/check that. Attaching a super-simple patch for the exporter that skips these flags for Fallout 3.