Hi, Austen.  You've obviously never been involved in barbershop singing, where "blend" is defined in great detail (but not necessarily identically) for both men's and women's choruses for competition purposes.  They come as close as humanly possible to an objective definition, so that everyone is judged on a level playing field.

 

The quick and dirty definition (although it is a negative one) is that in a good blend no individual voices stick out or call attention to themselves.  The individual voices "blend" to create a whole that's greater than the sum of its parts.  That's easy with a chorus of 200, very difficult with ensembles of 20 or fewer.  The King's Singers is one excellent example with one voice on a part, Chanticleer with more than one.  It's possible to get infinitely more detailed than that, but not really necessary.  I had one lead voice in my barbershop chorus, for example, with a vibrato that sounded out of place and cost us points in competition.  But it can be voice quality, intonation, volume, or anything else, not just inappropriate vibrato.

 

Balance, at least in my experience, is quite different, but equally important, and tends to be a matter of balance between sections, not individuals.  And again, in barbershop judging it is treated separately from blend.

 

Perhaps it's like Louis Armstrong's response to someone who asked him to define jazz:  "If you have to ask, you'll never understand!"

All the best,

John