  We present results from the first dedicated study in the time domain of the
hard X-ray variability behavior of blazars on long timescales based on $\sim$13
years of continuous hard X-ray data in the 14-195 keV band. We use
monthly-binned data from the recent 157-month Swift-BAT catalog to characterize
the hard X-ray variability of 127 blazars and search for potential differences
between the variability of BL Lacertae objects (BL Lacs) and flat-spectrum
radio quasars (FSRQs). A significant portion of the blazars in the sample
($\sim$37%) do not show statistically significant hard X-ray variability on
monthly timescales, which is deeply at odds with previous studies that show
that blazars are highly variable in the X-rays and other energy bands on a wide
range of timescales. We also find that, on average, the FSRQs and BL Lacs for
which we do detect variability exhibit similar flux variability; this suggests
that the variability in these FSRQs is not necessarily driven by variations in
the source function of scattered external radiation arriving from extended
regions, and that it is instead possibly driven by processes that lead to
variations in particle injection. In addition, only five blazars in our sample
show significant spectral variability in the long-term light curves. For 3
blazars, we find that a power law that changes slope on monthly timescales is
sufficient to characterize the variable hard X-ray spectrum, suggesting that,
at least for some bright blazars, the long-term spectra in the hard X-rays may
be described in a relatively simple fashion.
