We study the size-dependent exciton fine structure in monolayer black phosphorus quantum dots (BPQDs) deposited on different substrates (isolated, Si, and SiO$_2$) using a combination of the tight-binding method to calculate single-particle states and the configuration interaction formalism to determine the excitonic spectrum. We demonstrate that the substrate plays a dramatic role in the excitonic gaps and excitonic spectrum of the QDs. For reasonably high dielectric constants (\epsilon_{sub}∼\epsilon_{Si} = 11.7\epsilon_0$), the excitonic gap can be described by a single power law EX(R)=E(bulk)X+C/Rγ. For low dielectric constants $\epsilon_sub≤\epsilon_SiO_2 = 3.9 \epsilon_0$, the size dependence of the excitonic gaps requires the sum of two power laws EX(R)=E(bulk) g+A/Rn−B/Rm to describe both strong and weak quantum confinement regimes, where A, B, C, γ, n, and m are substrate-dependent parameters. We also predict that the exciton lifetimes exhibit a strong temperature dependence, ranging between 2–8 ns (Si substrate) and 3–11 ns (SiO2 substrate) for QDs up 10 nm in size.