I agree about a deficiency on their team. If it's a problem with too much sweat building up on his socks, then he should get better technology here or even change socks at the changeover.
As far as why Djokovic doesn't have blister problems, perhaps he doesn't sweat as much. There are considerable variance in the amount that individuals sweat.
4 x IG Prestige MP, 70 cm, 376 grams, 386 SW, ALU Power @50