The main goal of this paper is to establish necessary and sufficient conditions for the nonexistence of a global solution to the semilinear heat equation with a mixed local--nonlocal operator -Δ+ (-Δ) ^σ, under a general time-dependent nonlinearity. Our results complement the recent work of Carhuas-Torre et al. ArXiv, (2025), arXiv: 2505. 20401, in which the authors provide sufficient conditions for the existence and nonexistence of global solutions. In particular, our results recover the critical Fujita exponent for time-independent power-type nonlinearities, as obtained by Biagi et al. Bull. London Math. Soc. (2024), 1--20 and Del Pezzo et al. Nonlinear Anal. 255 (2025), 113761.
Kumar et al. (Fri,) studied this question.