The 18-year-old, who cemented his future with the U’s when signing a long-term four-year deal earlier this season, will switch to the Essex outfit to gather further experience of senior football having already made six appearances for United’s first-team.
First joining Cambridge as a 12-year old, he has also spent time in the Newmarket Town academy.
He made his first-team debut in the EFL Trophy encounter with Scunthorpe United last season and subsequently enjoyed a month`s loan spell with then-National League South side Bishop`s Stortford.
He then spent from September to October 2017 on loan with East Thurrock`s league rivals Hampton & Richmond Borough, making three starts.
Darling will be eligible to feature in his first game for the Rocks when they travel to Bognor Regis Town on Saturday.