Photon-counting distribution of fluorescence from a single molecule is modeled as a doubly stochastic Poisson distribution. A two-state model is considered where fluorescence from a dye molecule jumps between an 'on' (= high) intensity level and an 'off' (= low, background) level. Those intensity jumps may be caused by sojourns of a molecule into a non-emissive triplet state. Illustrative examples of photon-counting distribution are calculated for three parameter regimes. In the slow blinking regime, the 'on' and 'off' lifetimes are much longer then the integration time (= bin time). In the intermediate regime at least one of the 'on' and 'off' Lifetimes is comparable with the integration time. In the fast blinking regime, the 'on' and 'off' lifetimes are shorter then the integration time. The photon-counting distribution can be helpful in establishing whether an experimental intensity histogram is consistent with triplet blinking.