Let ${\cal M}^{\rm st}$ (${\cal M}^{\rm pst}$) be a moduli space of stable (polystable) bundles with fixed determinant on a complex surface with $b_1=1$, $p_g=0$, and let $Z\subset {\cal M}^{\rm st}$ be a pure $k$-dimensional analytic set. We prove a general formula for the homological boundary $\delta[Z]^{BM}\in H_{2k-1}^{BM}(\partial\hat {\cal M}^{\rm pst},\mathbb{Z})$ of the Borel-Moore fundamental class of $Z$ in the boundary of the blow up moduli space $\hat {\cal M}^{\rm pst}$. The proof is based on the holomorphic model theorem proved in a previous article, which identifies a neighborhood of a boundary component of $\hat {\cal M}^{\rm pst}$ with a neighborhood of the boundary of a ``blow up flip passage". We then focus on a particular instanton moduli space which intervenes in our program for proving the existence of curves on class VII surfaces. Using our result, combined with general properties of the Donaldson cohomology classes, we prove incidence relations between the Zariski closures (in the considered moduli space) of certain families of extensions. These incidence relations are crucial for understanding the geometry of the moduli space, and cannot be obtained using classical complex geometric deformation theory.