A choir blends when you can't pick out one voice: each section sounds like one singer, and the sections together sound like one instrument. Blending isn't singing quietly or hiding. It's listening to the people around you and fitting your sound to theirs.
Why it's worth the work
A blended section is easier to tune, because singers who match each other can hear when they're not quite together. It sounds fuller, because the voices add up instead of competing. And it sounds like what most people love about choirs: many voices, one sound.
How to blend
Six habits, in the order most singers learn them.
Amazing GraceGetting the voices ready...
Tap a part to hear it on its own
All of Amazing Grace, from "Amazing grace, how sweet the sound that saved a wretch like me.", in its parts: the tune with the others under it. Your part is the Soprano. Press a voice to take it out of the harmony or bring it back.
Listen more than you hear yourself
If all you can hear is your own voice, you're too loud for the section. Sing at a level where you can hear yourself and the singers either side of you. A good test: if you can't hear your neighbour, sing a little less until you can.
Match the vowels
Vowels are most of the sound a choir makes, and they're where blend is won or lost. Two singers on the same note with different "ah" vowels sound like two people. Shape each vowel the way your section does: listen to the singer next to you and copy the shape of their mouth and the colour of their sound, brighter or darker, rounder or more open.
Match the volume of your section
Every section has a level the director asks for. Sing inside it. One voice above the rest pokes out of the sound even when it's singing well, and the rest of the section can't fix that by singing louder.
Breathe, start and stop together
Blend is also timing. Breathe where your section breathes (mark the breaths in your copy), start on the director's gesture, and put final consonants in the same place: a "t" or "s" from twenty singers at twenty different moments sounds like a hiss. Agree where each one goes, and place it together.
Keep your top notes light
High notes are where blend usually breaks: one singer pushes, and suddenly the section has a soloist. Near the top of your range, use less weight and more of your head voice. Lighter carries more than you think in a choir.
Sing in tune with the chord
Two voices slightly apart in pitch beat against each other, and no amount of vowel matching hides it. Practise holding your note steady against a fixed one: the free drone holds the key note while you sing, and the tuner shows where your pitch sits. In a chord, your note may sit a little off the piano's: the just intonation tuner shows where it rings. The guide to singing in tune goes further.
Blend in a small group
In a quartet or an a cappella group with one voice on a part, blend works the same way, but you are the whole section. Balance matters even more: the melody usually leads, and the other parts sit just under it so the chord sounds full without covering the tune.
Practise with Stackapella
Blending starts with knowing your part so well that you can listen to everyone else. On each song's page you can hear every part and tap a part to hear it on its own; in the app you sing yours with the others playing. Try Amazing Grace: learn the alto part on its own, then sing it with all four parts and listen for the chord rather than your own voice.