Since acid blend is a combination of tartaric, citric, and malic acids, lemon juice isn't really a good substitute for it. You can wait and add it later, although it is generally added to the primary. After you taste the must, if it needs some acid, you can either go ahead and add the acid blend or some lemon juice if you think it's low on citric acid. I think the commercial acid blends are generally like 50% citric acid, 25% malic acid, and 25% tartaric, but those vary with manufacturer.
I wouldn't necessarily add the type of acid associated with certain fruit- for example, apple wine. Adding more malic acid might make the malic acid flavor too strong, but acid blend might bring a depth of flavor missing, since it's also adding citric and tartaric acid. Acid blend is just a generally "probably make it taste better" addition, and no harm will come without.