Is it obligatory for a woman to wear the hijab in her home, even in the absence of mahrams and visitors, to increase blessings, and is she sinful if she refuses to do so?

There is nothing in Islamic law that commands a woman to wear the hijab when she is alone in her home or with her husband. A husband's demand for this falls under extremism that violates both Sharia and natural inclination (fitra), because Allah has permitted a woman to adorn and beautify herself for her husband, and this is a cause of love and intimacy between them. Furthermore, wearing the hijab at home prevents a woman from adorning and beautifying herself. And Allah knows best.