During a voyage through Italy (1847) he visited the Kircher Museum, and his intercourse with G. B. de Rossi determined him to undertake in France the scientific work which the founder of Christian archeology had undertaken in Rome.
As early as 1848 Le Blant was commissioned to collect the inscriptions of the earliest days of Christianity in Gaul, and like de Rossi, he made an investigation of manuscripts, printed books, museums, churches, and the Gallo-Roman cemeteries.
The second volume of the work (Paris, 1865) obtained for its author his election as a member of the Académie des Inscriptions et Belles Lettres.
In the introduction he treats of the form, ornamentation, and iconography of these monuments; he dwells upon the relationship between the sarcophagi of Arles and those of Rome, and the difference between them and those of the south-west of France, in which he finds more distinct signs of local influence.
These studies were crowned by his work "Persécuteurs et Martyrs" (Paris, 1893), in which he displays his knowledge of history and his deep Christian convictions.