  A key method to produce trapped and laser-cooled molecules is the
magneto-optical trap (MOT), which is conventionally created using light
red-detuned from an optical transition. In this work, we report a MOT for CaF
molecules created using blue-detuned light. The blue-detuned MOT (BDM) achieves
temperatures well below the Doppler limit, and provides the highest densities
and phase-space densities reported to date in CaF MOTs. We observe short BDM
lifetimes at high magnetic field gradients, preventing magnetic compression as
a means to increase densities. By directly measuring the BDM restoring force,
we find that the short lifetimes are explained by low effective trap depths.
Notably, we find sub-mK depths at typical magnetic gradients, in contrast to
$\sim 50\,\text{mK}$ depths in red molecular MOTs and $\sim0.5\,\text{K}$
depths in red atomic MOTs.
