Questions are good ;-)
Reading between the lines of your description, the "blank" is the Transmission Reference (i.e. the clear container with only water). The light source + reference + spectrometer setup is then adjusted to give a maximum measurement value -- but w/o signal clipping (no signal distortion). They further appear to specify taking the measurement data at 530nm wavelength (you'll know this value once the spectrometer has been calibrated for measurement wavelength using the sharp spectral peaks of the CFL light source spectra). [The CFL source is ONLY used for wavelength-cal, NOT for measurements -- for that, use a broadband source.]
The spectrometer has a usable range of about 400-650nm so the "light red" color at 580 is probably a good fit to the beet juice. By starting with water (the reference with max signal) then introducing a beet juice sample, a reduction in measured signal would be expected. You can collect the data of the entire spectrum so you can see how various colors are affected but only the value at 580 will be reported I assume. If the signal from a sample drops by 90%, then the Transmission is 10%.
Absorbance (A) is calculated as: A = 2 - log10(T%). So, for 10% transmission, the Absorbance = 2 - log10(10) = 2 - 1 = 1. The Absorbance is a Log scale which is useful for scientific plots. [ The '2' is there because for water, T=100% and log10(100) = 2 which means A=0; zero absorbance.] However, it sounds like they are asking for Absorption Percentage and Transmission Percentage so it would be good to check what calculation for Absorption is being requested.
The Lambert-Beers Law states the Absorbance of light through a uniform fluid is A = ELC (E (epsilon) is a unique coefficient, L = length of the optical path and C = concentration. So this means that if the container is twice as thick, or the sample concentration were twice as dense, the Absorption would be doubled -- a linear relationship.
Again, this is just some related information. Hopefully the experiment's documentation will help clarify what calculation is expected.