When making an absorption spectrum, why must ou re-blank at every wavelength?

Answer 1

Because the absorption of your blank changes depending on the wavelength of light you're using, just as does the absorption curve of your sample. so, in order to get the absorption of the sample at different wavelengths, you have to be able to subtract the blank at those same wavelengths.

Answer 2

Because the detector `s response is dependent on wavelength and the solvent were the sample is disolved has its own absorbtion characteristics .By re blanking you are canceling these properties.