Could anyone point me to some manual or tutorial, possibly step-by-step one, on reconstruction of (quasi-) bolometric light curve using UBVRI-photometry data?
Both links and names are fine for me.
If you want a bolometric light curve from photometric points, you don't need to reconstruct its SED inside the range you already have data points. Use effective central wavelength of each band with its magnitude. Do trapezoid. This is an approximation, but it is better than reconstructing the SED that makes you rely more on assumptions for most circumstances, especially that you have UBVRI.
If, under some circumstances, you really need some more data points in between, or you really want a SED, blackbody is the first-order approximation, and is commonly used in SNe (at young ages). Less known issue in optical VBRI. There might be some issues with U like UV excess/suppressed. In this case, just fit the data to a blackbody.