The commonly white MoO3 with negligible light absorption is successfully converted to a black, strongly light-absorbing material through heavy V-doping, leading to over 90% absorption from 250 to 2000 nm. Experiments and calculations reveal that the carrier concentration in MoO3 and the imaginary part of the dielectric function increase significantly with increasing V-doping concentrations. The V-doped MoO3 nanospheres are embedded densely into a porous interlaced poly(vinyl alcohol) (PVA) network for solar seawater desalination. The harvested light is efficiently converted to local heat for interfacial seawater evaporation due to the strong light absorption of the doped nanospheres, the reabsorption of the scattered light by the closely packed nanospheres, and the low thermal conductivity of the PVA network. The solar absorber film containing 33.3 mol % V-doped MoO3 nanospheres gives a fast seawater evaporation rate of 2.01 kg m-2 h-1 and a high solar energy conversion efficiency of 93.44%. This work shows the great potential of turning non-light-absorbing dielectric oxide semiconductors into strongly light-absorbing materials through heavy doping for photothermal conversion applications. The design concept for solar absorber films with closely packed light-absorbing nanospheres will also inspire future developments of solar-powered water management applications.