[Utah-astronomy] In the blue

I was just reading an article about the Herschel 400. The author Jay Reynolds Freeman made this statement: I have informally known for a long time that many older cataloged magnitudes of deep-sky objects systematically underestimate their brightness to the visual observer--the fact that many magnitudes were obtained in blue wavelengths guarantees that result... What does it mean "obtained in blue wavelengths "? How are magnitudes obtained in the blue wavelengths? Jim