  Late bloomers are massive ($M_* >10^{10}\,M_\odot$) galaxies at $z<1$ that
formed the majority of their stars within $\sim$2 Gyr of the epoch of
observation. Our improved methodology for deriving star formation histories
(SFHs) of galaxies at redshifts $0.45 < z < 0.75$ from the
Carnegie-Spitzer-IMACS Survey includes confidence intervals that robustly
distinguish late bloomers from "old" galaxies. We use simulated SFHs to test
for "false positives" and contamination from old galaxies to demonstrate that
the late bloomer population is not an artifact of our template modeling
technique. We show that late bloomers account for $\sim$20% of $z\sim0.6$
galaxies with masses of the modern Milky Way, with a moderate dependence on
mass. We take advantage of a 1% overlap of our sample with HST (CANDELS)
imaging to construct a "gold standard" catalog of 74 galaxies with
high-confidence SFHs, SEDs, basic data, and HST images to facilitate comparison
with future studies by others. This small subset suggests that galaxies with
both old and young SFHs cover the full range of morphology and environment
(excluding rich groups or clusters), albeit with a mild but suggestive
correlation with local environment. We begin the investigation of whether late
bloomers of sufficient mass and frequency are produced in current-generation
$\Lambda$CDM-based semi-analytic models of galaxy formation. In terms of halo
growth, we find a late-assembling halo fraction within a factor-of-two of our
late bloomer fraction. However, sufficiently delaying star formation in such
halos may be a challenge for the baryon component of such models.
