We report the sub-parts-per-billion (sub-ppb) level detection of lead using black phosphorus (BP) integrated tilted fiber grating (BP-TFG) configuration where the light-matter interaction is enhanced. We synthesized BP nanosheets by using a liquid-phase exfoliation method and developed an in-situ layer-by-layer (i-LbL) technique for BP deposition. These approaches secured high-quality BP coating on specific fiber cylindrical surface with strong adhesion as well as a prospective thickness control. By taking advantage of i-LbL deposition, the unique optical tunable features including coating thickness-dependent polarization and polarization-selective coupling have been experimentally observed. Furthermore, the BP-TFG was exploited as the first BP-fiber optic chemical sensor for heavy metal Pb²⁺ ions detection, demonstrating significant performance of ultrahigh sensitivity up to 0.5 × 10⁻³ dB/ppb, lower limit of detection down to 0.25 ppb, and wider concentration range from 0.1 ppb to 1.5 × 10⁷ ppb. We believe that this BP-fiber optic architecture opens the path as an optical platform for superior chemical sensing and biomedical applications.