  High magnetic field $^{63,65}$Cu NMR spectra were used to determine the local
spin polarization in the 1/3 magnetization plateau of azurite,
Cu$_3$(CO$_3$)$_2$(OH)$_2$, which is a model system for the distorted diamond
antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the
Cu2 (dimer) sites is found to be field independent, negative and strongly
anisotropic, corresponding to $\approx$10 % of fully polarized spin in a
$d$-orbital. This is close to the expected configuration of the "quantum"
plateau, where a singlet state is stabilized on the dimer. However, the
observed non-zero spin polarization points to some triplet admixture, induced
by strong asymmetry of the diamond bonds $J_1$ and $J_3$.
