Cosmological Brightness–Intensity Transport Bias (CBITB) is a phenomenological framework that addresses the inference of cosmic acceleration from observational data. The work does not question the cosmological expansion or the geometric origin of redshift. Instead, it examines whether the commonly inferred accelerated expansion necessarily reflects a dynamical property of spacetime, or whether it may arise from a minimal, systematic bias in the transport or interpretation of observed intensity. The term intensity is used in an operational sense and explicitly includes both absolute brightness and the temporal structure of observed light curves. CBITB introduces no new spacetime dynamics, no modified gravity, no additional energy density, and no microscopic interaction mechanism. It is formulated entirely at the level of observational inference and data interpretation, under strict constraints from achromaticity, image sharpness, surface brightness relations, and CMB spectral integrity. The document develops the idea from a conceptual inconsistency, traces the logical constraints imposed by established observations, and arrives at a minimal bias model that remains locally unobservable but can accumulate over cosmological distances. Within this framework, cosmic acceleration appears as an inference artifact rather than a direct observational fact. A key contribution is the proposal of a concrete, falsifiable observational test: the Intensity Differential Dilation Index (IDDI), based on differential time‑structure comparisons of multiple lensed images of the same variable source. The test is deliberately designed to be raw‑data‑oriented, conservative, and independent of any specific microscopic explanation. This document is not intended as a completed theory, but as a carefully delimited thought framework and test proposal. Its purpose is to make the underlying reasoning transparent and accessible to readers with a background in physics, astronomy, or related fields, and to invite further scrutiny, refinement, or refutation by those working with relevant observational data.