A way to search for dark matter is to look for an excess in the cosmic positron spectrum above 1 GeV. In that context, an enhancement of the signal due to dark matter substructures is often introduced and referred to as the boost (or clumpiness) factor. Recent studies show not only that the boost factor does depend on energy, but is a statistical property of the distribution of substructures inside the Milky Way. Bertone's scenarios in which a relatively small number of intermediate-mass black holes are present in the Milky Way are investigatedhere. For m_chi = 100 GeV, boosts of order 103 are found, with a high dispersion, especially near the source energy.