I believe this portrait shows Maria Alexandrovna dressed to mourn the loss of her oldest son Nicholas in 1865. The Hermitage dates this to the 1870s and they know more then I do, but I'll stand on my estimate based on what shows of her dress. It looks 1860s to me...
Keywords: Tsaritsa Maria Alexandrovna, Hesse family, Romanov family, Tsaritsa, Russian, curly coiffure, veil, floral lace headdress, attifet, high neckline, ruff, long sleeves, brooch