You simply cannot interpolate between geometries with different polycounts.
Continuous LOD is old-tech though, as it is slow because it requires new geometry to be sent to the GPU every frame
You should just have a couple of LODed versions of your model, and switch among them at the proper distance. You know you finetuned the poly-count/distance properly, when you can't see the switch anymore.
The most recent GPUs can do c-lod, but you might wonder whether that's worth the effort of developing when the alternative is so plain simple.
Do you know some tools permitting to reduce the number of polygons in a model (is there a feature in Blender pemitting that ?), cause if there's not I'll have to ask my artists to make again their models in low-poly, then smooth/correct them gradually for higher-polygons versions.