Supposing a type with the following definition:

public final class StringWrapper {
    private final Optional<@NotBlank String> num;

    public StringWrapper(Optional<String> num) {
        this.num = num;
    }
}

I would expect Optional.empty() to generate no validation errors, because the value is not present. Certainly that's the way other constraints work. Unfortunately, what I get instead is:

'must not be blank'

It's pretty clear why this is happening:

@Override
public void extractValues(Optional<?> originalValue, ValueExtractor.ValueReceiver receiver) {
    receiver.value( null, originalValue.isPresent() ? originalValue.get() : null );
}

...and since null isn't a valid value, it of course fails the validation. Any null-failing validator would complain at that point.

A more idiomatic way to deal with optionals would be something like:

@Override
public void extractValues(Optional<?> originalValue, ValueExtractor.ValueReceiver receiver) {
    originalValue.ifPresent( value -> receiver.value( null, value ));
}

...which would only call the receiver if it was present. However, that's only if the value was present, which is a behavior change, so I'm unsure of all the ramifications of that.