To my knowledge, there has been but one published, peer-reviewed study of carao for anemia. It was done in 2004 by the Centro de Investigación y Desarrollo de Medicamentos, and titled Actividad antianémica de la Cassia grandis L. (Translation: Anti-anemic Activity of Cassia Grandis L.)
The link to the original article is now broken (http://bvs.sld.cu/revistas/far/vol38_3_04/far09304.htm), however I have a full copy in the original Spanish here. The organization's website is now https://instituciones.sld.cu/cidem/productos-informativos/
The summary in English, provided by the authors, is below:
"The population of the eastern zone of Cuba refers to the beneficial effects of the traditional use of Cassia grandis L. in anemia by using dry powder obtained from the fruit as a nutritional supplement. The objective was to evaluate this effect in an experimental model of iron-deficiency anemia in rats, induced by successive blood extractions and the administration of an iron-lacking diet. All the animals were on a semisynthetic iron-deficiency diet and blood was extracted 3 times a week until attaining haemoglobin concentrations in blood under 9 g/dL. They were divided into 3 groups on the same diet: group I with no supplement, group II supplemented with 15 mg of iron/kg of diet, and group III the same amount of iron plus 750 mg/kg of body weight of dry powder of Cassia grandis L. for other15 days. At the end, the concentrations of iron, hemoglobin and hematocrit in blood were determined. The mean concentrations of hemoglobin 15 days after treatment were sigificantly different in the 3 experimental groups with better results in the group that was supplemented iron and Cassia grandis L. In this group, it was observed a marked increase of the mean values of iron in plasma compared with the values obtained in the non-supplemented animals and in the animals that received only iron in the diet. The hematocrit percentage did not show any significant difference between treatments. The results corroborated the popular and traditional use of Cassia grandis L.in the anemic states on improving the utilization of iron and the production of hemoglobin."