The main role of blood plasma is to transport proteins, hormones, and nutrients to certain parts of the body. All blood constituents are carried by blood plasma through the circulatory system. Cells get rid of waste products into the plasma. In this paper, we present a surface plasmon resonance (SPR) biosensor based on a black phosphor (BP) layer to improve the sensor performance. The black phosphor layer is employed as an interacting medium with the sensing medium for the improvement of the sensor sensitivity. The sensor is employed for the detection of blood plasma. Four metals are used: silver (Ag), gold (Au), copper (Cu), and aluminum (Al). We found that structures with the BP layer have better performance than those without a BP layer. Cu-structure has shown the highest sensitivity while the Ag-structure has shown the highest quality factor and detection accuracy and the lowest FWHM. As the concentration of the plasma increases, the sensitivity can be enhanced.