  We present observations of SN 2022joj, a peculiar Type Ia supernova (SN Ia)
discovered by the Zwicky Transient Facility (ZTF). SN 2022joj exhibits an
unusually red $g_\mathrm{ZTF}-r_\mathrm{ZTF}$ color at early times and a rapid
blueward evolution afterward. Around maximum brightness, SN 2022joj shows a
high luminosity ($M_{g_\mathrm{ZTF},\mathrm{max}}\simeq-19.7$ mag), a blue
broadband color ($g_\mathrm{ZTF}-r_\mathrm{ZTF}\simeq-0.2$ mag), and shallow Si
II absorption lines, consistent with those of overluminous, SN 1991T-like
events. The maximum-light spectrum also shows prominent absorption around 4200
\r{A}, which resembles the Ti II features in subluminous, SN 1991bg-like
events. Despite the blue optical-band colors, SN 2022joj exhibits extremely red
ultraviolet minus optical colors at maximum luminosity ($u-v\simeq0.6$ mag and
$uvw1 - v\simeq2.5$ mag), suggesting a suppression of flux at $\sim$2500--4000
\r{A}. Strong C II lines are also detected at peak. We show that these unusual
spectroscopic properties are broadly consistent with the helium-shell double
detonation of a sub-Chandrasekhar mass ($M\simeq1 \mathrm{M_\odot}$)
carbon/oxygen (C/O) white dwarf (WD) from a relatively massive helium shell
($M_s\simeq0.04$--$0.1 \mathrm{M_\odot}$), if observed along a line of sight
roughly opposite to where the shell initially detonates. None of the existing
models could quantitatively explain all the peculiarities observed in SN
2022joj. The low flux ratio of [Ni II] $\lambda$7378 to [Fe II] $\lambda$7155
emission in the late-time nebular spectra indicates a low yield of stable Ni
isotopes, favoring a sub-Chandrasekhar mass progenitor. The significant
blueshift measured in the [Fe II] $\lambda$7155 line is also consistent with an
asymmetric chemical distribution in the ejecta, as is predicted in
double-detonation models.
