Aims: This is the first paper of a series devoted to studying the population of blue massive stars in NGC 55, a galaxy of the Sculptor group at a distance of about 2 Mpc. Methods: We have obtained optical (3300{-}6210 Å), low-resolution spectra of approximately 200 blue massive stars with VLT-FORS2, which we have classified with the aid of Milky Way and Magellanic Cloud standard stars. Results: We present the first census of massive blue stars in NGC 55. A study of stellar radial velocities shows agreement with existing H I rotational velocity curve work and reveals the presence of one object with peculiar velocity. A qualitative study of the stellar metallicity suggests that its global distribution over NGC 55 is close to that of the LMC, as derived from previous studies. Conclusions: We present a catalogue with 164 classifications of blue massive stars in NGC 55. This catalogue is a first and necessary step for the subsequent quantitative study of blue massive stars in NGC 55 with state-of-the-art model atmospheres. Based on observations obtained at the ESO VLT for Large Programme 171.D-0004. Appendices A-C are only available in electronic form at http://www.aanda.org