I think when folks use the word "blend" pertaining to the vocalizer...they are referring to the volume level compared to the lead voice...

Over bearing harmony, just does not sound natural and distracts from the lead voice..

To low of a contrast harmony level..does not sound realistic....but the proper "blend" can make both the harmony and lead sound real (or as close to real as possible with the technology).

The on board vocalizer on the Yamaha products I have owned...were not stable enough for me..it would distort to easily and was inconsistent (a mind of it's own)..Even the lead mic input seemed to have problems with signal level...

It seems that Frank is saying this is improved, but still not able to compete with the high quality stand a lone units..

Using a harmony correctly certainly includes ..knowing when to use it or not within a song...Overuse can ruin an entire performance ..all night long...sparingly can dramatically make the over all performance better..

When it doubt..don't use it..