  The masses of the black holes powering quasars represent a fundamental
parameter of active galaxies. Estimates of quasar black hole masses using
single-epoch spectra are quite uncertain, and require quantitative improvement.
We recently identified a correction for C IV $\lambda$1549-based scaling
relationships used to estimate quasar black hole masses that relies on the
continuum-subtracted peak flux ratio of the ultraviolet emission-line blend Si
IV + OIV] (the $\lambda$1400 feature) to that of C IV. This parameter
correlates with the suite of associated quasar spectral properties collectively
known as "Eigenvector 1" (EV1). Here we use a sample of 85 quasars with
quasi-simultaneous optical-ultraviolet spectrophotometry to demonstrate how
biases in the average EV1 properties can create systematic biases in C IV-based
black hole mass scaling relationships. This effect results in nearly an order
of magnitude moving from objects with small $<$ peak $\lambda$1400/C IV $>$,
which have overestimated black hole masses, to objects with large $<$ peak
$\lambda1400$/C IV $>$, which have underestimated values. We show that existing
reverberation-mapped samples of quasars with ultraviolet spectra -- used to
calibrate C IV-based scaling relationships -- have significant EV1 biases that
result in predictions of black hole masses nearly 50\% too high for the average
quasar. We offer corrections and suggestions to account for this bias.
