Recent research studies demonstrated that heteropoly blues (reduced polyoxometalates) could act as a special kind of solid material with potential functions in antiviral activity, and photocatalytic, photothermal and semiconducting properties. In this work, we develop a “directed precursor self-assembly” strategy for the facile synthesis of heteropoly blues (HPBs), so four representative HPBs [GeW₁₀Mo^V₂O₄₀], [GeMo^VI₁₀Mo^V₂O₄₀], [P₂W₁₆Mo^V₂O₆₂] and [P₂W₁₂Mo^VI₂Mo^V₄O₆₂] have been synthesized and structurally characterized. These four heteropoly blue compounds were synthesized by the solution self-assembly reaction of a precursor [Mo^V₂O₄(H₂O)₂(ox)₂]²⁻ with vacant polyoxometalates. Electrospray ionization mass spectrometry (ESI-MS) was used to analyze the assembly mechanism of these HPBs. A detailed study on the magnetic properties was also carried out, which shows that the number of Mo^V and their locations are capable of adjusting the electron distribution in HPBs.