Hey BoUBoU -
Here is the basic setup you would need to do in the Event Graph to achieve the transition you are looking for. In the material of the object in the blueprint (for me it was Cylinder 0) all you need to do is set up a LERP between your two colors or textures and use a scalar parameter in a Material Parameter Collection as an Alpha.
Thank You
Eric Ketchum
EDIT: There is also materials included in Starter Content and the Content Examples which use a distance calculation to blend for LOD purposes which could be modified to also drive this same logic.