This sounds refreshingly like a realistic rumour, with Ajax almost certainly willing to sell Blind at a reasonable price, and United badly in need of a back-up -- or a challenger -- to Luke Shaw on the left. At present, Reece James is his only natural competitor for the left wing-back slot, and though James has starred in pre-season, he's unlikely to see much first team action this season.
Blind fits the bill perfectly, having impressed in that very role in Louis van Gaal's system at the World Cup in Brazil. He's not the quickest, though his smart positioning and excellent crossing more than make up for his lack of pace. Blind is also capable of playing in central midfield or at the centre of defence, making him a useful utility man for positions United could do with reinforcing.
In short, signing Blind would make an awful lot of sense.