Massive stars play a pivotal role in shaping the Universe. However, comparisons between observational data and theoretical models of massive stars have given rise to long-standing and new discrepancies that have questioned our understanding of these objects. A main uncertainty arises from the overdensity of B-type supergiants (BSGs) located beyond the terminal-age main sequence, as predicted by the state-of-the-art evolutionary models. It remains unclear whether the width of the main sequence has to be redefined or if the overdensity is the result of overlapping populations following different evolutionary paths. Here, we outline some key findings from the first author's Ph.D. thesis, which aims at improving this situation by carrying out a holistic empirical study of the largest spectroscopic sample of BSGs analyzed to date. The results provide important clues about the nature of these objects and establish new constraints on the evolutionary models of massive stars.