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The quest for dormant stellar-mass black holes (BHs) in massive binaries (i.e. comma OB+BH systems) is raging. All the candidates proposed so far have been heavily disputed in the weeks following their report. Combining a literature search and archival spectroscopic data with innovative analysis techniques comma we have identified two promising massive binaries harboring unseen companions with masses above the BH-mass limit comma and for which simulations could discard the presence of a 5Msun main-sequence stellar companion. Here we request STIS-FUV spectroscopy of the best candidate (that with the heaviest companion mass) to unveil or discard the presence of a hot comma Helium stripped star as the unseen companion. A Helium stripped star would indeed contribute very little to the system optical flux comma but its contribution would become more significant in the far-UV. Should the proposed observations fail to reveal such hot companion comma this project will prove beyond doubt the presence of a BH comma with important consequences on the evolution of gravitational-wave progenitors and the debated presence of a kick accompanying BH formation. Either outcome -- either a Helium stripped star or a dormant BH -- will result in a significant leap forward in our understanding of the evolution of massive stars comma but only the FUV capabilities of HST can discriminate between both scenarios.