Hemosiderosis occurring just in lower limbs is not known and is condition in which iron builds up in the lungs.
Pulmonary Hemosiderosis, while uncommon, can occur in association with a wide variety of clinical disorders, many of which have overlapping features of glomerulonephritis and immune complex diseases, like systemic lupus erythematosus. While lung involvement has to be a consistent feature, in very rare case there might not be severe pulmonary symptoms. Nonspecific recurrent or chronic pulmonary symptoms, such as cough (dry or productive), dyspnea, tachypnea, and wheezing, are observed in most patients. Read here;
With the history of prostatic cancer in your husband, one needs to exclude local extension of the disease in pelvis which may be hampering the blood circulation of the lower limbs (feet and legs). A pelvic ultrasound study and Colored Doppler studies of the blood vessels can be more informative about the causative of these features.
Your follow up questions are welcome.
Yes, Hemosiderosis diagnosis will need evaluation of lung functions and some immunological tests and blood work up.
The black color of the limbs and deposition of the iron in the tissues can be due to excessive permeability of the blood vessels, which can occur in some immune mediated disease. Consultation with a rheumatologist or immunologist for this would be prudent and helpful.
Please feel free for your further questions.