“Harmony app” covers four different things, and the right one depends on your problem. We make one of them, so read the last section knowing that; the rest is as straight as we can make it.

Learning a part someone already wrote

Harmony Helper(iOS and Android, subscription) plays a song's arrangement, isolates your part, and gives real-time pitch feedback while you sing it. If you are in a choir or a musical and you need to learn your line from the score, this is the tool. It does not teach you to find a harmony line yourself.

Sing Your Part and How to Sing Harmony (Caleb Hugo) do the same for hymns, free, with sliders to mute the other parts. No grading.

Training the ear

EarMaster, Perfect Ear, ToneGym and Functional Ear Trainer are recognition trainers: hear it, name it. EarMaster and ToneGym have singing exercises, but they are single lines sung back, not a harmony held against a tune. Good for the theory, not the pull.

Seeing your pitch

Vocal Pitch Monitor, Nail the Pitch and Singing Carrots show your pitch on a line as you sing. Useful for finding out whether you go flat on long notes. None of them plays a second line for you to sing against.

Learning to sing harmony itself

This is what Blend does, and as far as we can find, nothing else does: it plays a melody, tells you which line to sing against it (a third above, a sixth below, a drone, a counterline, a canon), and grades every note through your microphone, in the browser. The lessons go from a stepwise tune with the staff shown to a leaping one by ear, and half the clips are duets with your own recorded voice. The first three stages are free with no account, and Pro is a monthly subscription.

If you already have the part written and need to learn it, get Harmony Helper. If you want to be the person who can make up a harmony on the spot, that is the skill Blend trains.