The magnetic properties of oriented CdSb single crystals doped with 2 at% of Ni are investigated. From measurements of magnetic irreversibility defined by deviation of the zero-field-cooled (ZFC) susceptibility from the field-cooled (FC) susceptibility, the value of the mean anisotropy field B_K ∼ 4 kG is obtained. The ZFC susceptibility displays a broad maximum at a blocking temperature, T_b, depending on B according to the law [T _b(B)/T_b(0)]^1/2 ≤ 1 - B/B_K with T_b(0) ∼ 100 K. The field dependence of the magnetization exhibits saturation above ∼20-30 kG with values of M_s different for B along the [1 0 0], [0 1 0] and [0 0 1] axes. The temperature dependence of M_s is weak, increasing slightly upon cooling the sample below ∼100 K. The temperature dependence of the coercive field, B_c(T), is weak above T_b but is enhanced strongly with decreasing temperature below T_b. The anisotropy of B_c is inverted with respect to the anisotropy of M_s. Such behaviour can be attributed to spheroidal Ni-rich Ni_1-xSb_x nanoparticles with a high aspect ratio, broad size distribution and distribution of the orientation of the major axis around a preferred direction. The relation B_c ≪ B_K and the anisotropies of M_s and B_c are consistent with reversal of the magnetization by the curling mode, whereas the T_b(B) dependence is typical of the coherent rotation mode. This difference is connected to the proximity of the average transversal cluster radius to a critical value for transition between the two magnetization reversal modes within a wide crossover interval, due to broad distribution of the cluster sizes.