Black metals are metals with a sponge-like nanoporous surface. Their surface appears black because light enters the pores and does not exit back due to multiple reflections. Black metals can be used as heat-absorbing coatings in thermal detectors or pyroelectric devices. In this study, 500 nm and 1000 nm thick black aluminium (B-Al) films were deposited on 1 mm thick fused silica substrates using pulsed DC magnetron sputtering in a mixed atmosphere of argon and nitrogen of 94% and 6% respectively. The resulting B-Al films had a soft surface which was easily susceptible to scratching. We observed that nanowires began to grow from scratches and small abrasions on the film surface when the films were annealed in a vacuum of 10-3 Pa at a substrate temperature of 350 °C. Energy-dispersive X-ray analysis confirmed that the atomic percentage of aluminium was 70–80% both in the film and the nanowires, and the remaining elements were carbon, nitrogen and oxygen. Nevertheless, no visual changes in the shapes of the mechanically intact pores were observed after annealing when examined with scanning electron microscopy.