Unveiling the Eigenvector 1 sequence in BZQ blazars – I. Mg ii and ultraviolet Fe ii emission in the UV main sequence

We investigate the location of flat-spectrum radio quasars confirmed as blazars (BZQ-C), Fanaroff–Riley type II radio-loud (RL) quasars (FRII-Q), and radio non-detected quasars within the ultraviolet (UV) Eigenvector 1 (E1) plane defined by the Mg ii line width (FWHM$_{{\rm Mg\, {\small II}}}$) and the UV Fe ii strength ($R_{\rm Fe \,{\small II}}$ UV). This represents the first systematic comparison of BZQ blazars with their proposed parent population in the UV E1 framework. To minimize selection biases, we construct statistically representative control subsamples and perform matched-subsample analyses in redshift and Mg ii luminosity. Population differences are assessed using one- and two-dimensional non-parametric tests, including the Kolmogorov–Smirnov, Anderson–Darling, Mann–Whitney U, and Fasano–Franceschini statistics. We find that the three active galactic nucleus (AGN) populations occupy statistically distinct regions of the UV E1 plane. The dominant separation is driven by FWHM$_{{\rm Mg\, {\small II}}}$, with BZQ blazars systematically exhibiting narrower line profiles than FR ii-Q quasars, while radio non-detected quasars lie between both populations. In contrast, $R_{\rm Fe\, {\small II}}$ UV provides only limited discrimination between the RL subclasses, although it separates radio-loud from radio non-detected quasars at the population level. The observed ordering demonstrates that the UV E1 plane preserves the physical sequence expected from RL AGN unification. Inclination-dependent projection provides a natural first-order explanation for the Mg ii line-width ordering. Additional differences in intrinsic BLR structure or accretion properties may contribute to the observed distributions, although the present analysis does not allow these effects to be separated independently from orientation.