  Spin defects in hexagonal boron nitride (hBN) are promising quantum systems
for the design of flexible two-dimensional quantum sensing platforms. Here we
rely on hBN crystals isotopically enriched with either $^{10}$B or $^{11}$B to
investigate the isotope-dependent properties of a spin defect featuring a
broadband photoluminescence signal in the near infrared. By analyzing the
hyperfine structure of the spin defect while changing the boron isotope, we
first unambiguously confirm that it corresponds to the negatively-charged
boron-vacancy center (${\rm V}_{\rm B}^-$). We then show that its spin
coherence properties are slightly improved in $^{10}$B-enriched samples. This
is supported by numerical simulations employing cluster correlation expansion
methods, which reveal the importance of the hyperfine Fermi contact term for
calculating the coherence time of point defects in hBN. Using cross-relaxation
spectroscopy, we finally identify dark electron spin impurities as an
additional source of decoherence. This work provides new insights into the
properties of ${\rm V}_{\rm B}^-$ spin defects, which are valuable for the
future development of hBN-based quantum sensing foils.
