While various studies suggest that informal statistical inference (ISI) can be developed by young students, more research is needed to translate this claim into a well-founded learning trajectory (LT). As a contribution, this paper presents the results of a cycle of design research that focuses on the design, implementation, and evaluation of the first part of an LT for ISI, in which ninth-grade students (N = 20) are introduced to the key concepts of sample, frequency distribution, and simulated sampling distribution. The results show that an LT starting from repeated sampling with a black box may support the accessibility of these concepts, as these students were able to make inferences with the frequency distribution from repeated samples as well as with corresponding simulated sampling distributions. This suggests a promising way to make ISI more accessible to students.