Production of watermelons for seed consumption is popular in many regions of the world. In the Mediterranean area, large white seeds are preferred. Six breeding lines were selected for investigating the nature of black spot, an undesirable feature reducing the commercial value of the seeds. Black spot is expressed as blackened areas of the seed coat, mostly near the margin in mild cases, but extending over much of the seed coat in severe cases. Sowing date had a significant effect on expression of black spot. Seeds that developed in early summer (June) had low frequency and severity of black spot expression, whereas seeds that developed later in the summer (July–August) had markedly increased expression. Large differences were also observed among the breeding lines. There was a significant negative correlation between severity of black spot and seed weight, suggesting that black spot is a stress-related phenomenon.