  Blue metal-poor stars (BMPs) are main sequence stars that appear bluer and
more luminous than normal turnoff stars. They were originally singled out by
using B-V and U-B colour cuts. Early studies found that a larger fraction of
field BMP stars were binaries compared to normal halo stars. Thus, BMP stars
are ideal field blue straggler candidates for investigating internal stellar
evolution processes and binary interaction. In particular, the presence or
depletion in lithium in their spectra is a powerful indicator as to their
origin. They are either old, halo blue stragglers experiencing internal mixing
processes or mass transfer (Li-depletion), or intermediate-age, single stars of
possibly extragalactic origin (2.2dex halo plateau Li). However, we note that
internal mixing processes can lead to an increased level of Li. Hence, this
study combines photometry and spectroscopy to unveil the origin of various BMP
stars. We first show how to separate binaries from young blue stars using
photometry, metallicity, and lithium. Using a sample of 80 BMP stars (T>6300K),
we find that 97% of the BMP binaries have V-Ks_0 < 1.08$\pm0.03$, while BMP
stars that are not binaries lie above this cut in 2/3 of the cases. This cut
can help classify stars which lack radial velocities from follow-up
observations. We trace the origin of two BMP stars from the photometric sample
by conducting a full chemical analysis using new high-resolution and high SN
spectra. Based on their radial velocities, Li, alpha, and s- and r-process
abundances we show that BPS CS22874-042 is a single star
(A(Li)$=2.38\pm0.10$dex) while with A(Li)$=2.23\pm0.07$dex CD-48 2445 is a
binary, contrary to earlier findings. Our analysis emphasises that field blue
stragglers can be segregated from single metal-poor stars, using V-Ks colours
with a fraction of single stars polluting the binary sample, but not vice
versa.(Abridged)
