Yes, you are right, polymer-polymer blends are the heterogeneous materials. So, the mechanical characteristics which are measured in course of indentation tests, will depend on size (for example, radius) of indenter (scale effect). This influence is determined by relation of  indenter tip size and characteristic size of polymer phase of blend microstructure (where will the indenter's tip get). As I remember, similar question relating to other kinds of composites including disperse-reinforced polymer materials and metallic alloys has been discussed previously in Research Gate.  

Best regards