Cross-validation is commonly used to select the recommendation algorithms that will generalize best on yet unknown data. Yet, in many situations the available dataset used for cross-validation is scarce and the selected algorithm might not be the best suited for the unknown data. In contrast, established companies have a large amount of data available to select and tune their recommender algorithms, which therefore should generalize better. These companies often make their recommender systems available as black-boxes, i.e., users query the recommender through an API or a browser. This paper proposes RecRank, a technique that exploits a black-box recommender system, in addition to classic cross-validation. RecRank employs graph similarity measures to compute a distance between the output recommendations of the black-box and of the considered algorithms. We empirically show that RecRank provides a substantial improvement (33%) for the selection of algorithms for the MovieLens dataset, in comparison with standalone cross-validation.