How does a spectrophotometer measure absorbance based on the intensity of detected light?
There are two possibilities:

If there is only one optical pathway, you record i) the blank sample for all wavelengths of interest and store the detector's intensity information in function of $\lambda$ in a working memory.  Light passing the blank is then assumed to equate $I_0$. Then you record ii) the wavelength dependent intensities $I_\mathrm{abs}$ with your sample of interest, and report the subsequently corrected data as spectrum $\mathrm{Abs} = f(\lambda{})$.

.or.

Your instrument permits the simultaneous recording of sample and blank to record both intensity data and reporting the already corrected absorption in one run.  Be sure, though, to assert which of the two paths (= two sample holders) is about your sample, and the blank (which may depend on spectrometer and software).