The Beer-Lambert law Absorbance = (extinction coefficent)(pathlength of light)(concentration) allows you to measure the absorbance of sample in a UV spec, and change the rate from absorbance units / time to change in concentration / time. the pathlength of light being the width of the cuvette and the extinctin coefficent being specific to the product molecule.
A blank is used in order to cancel out or zero the absorbance of all the other components in the sample except the component whose absorbance is to be measured. For example, if you want to measure the absorbance of a solute in water, you will use only water as a blank and the spectrophotometer will subtract the absorbance of water from the spectrum when you measure the absorbance of your solute in water.
Simply because we cannot measure light absorbed. We are, however, able to measure light transmitted through the use of a spectrophotometer. The device works by shining light of a specific wavelength on a substance and measuring the amount of light that gets through. This "transmittance" has a negative logarithmic relationship to absorbance.
The optimum wavelength is the wavelength by which the most light is absorbed by a substance. It can be found by finding the highest absorbance obtained when testing the substance's absorbance at various wavelengths. The wavelength that results in the greatest light absorbance is your optimum wavelength.