The reality TV star, 28, showed fans his thoughts were with Blac when he shared a photo of the model on his Instagram feed.
The shot, taken from a video call, showed Rob in the corner as the two talked online.
The cosy shot shows no indication of Blac's earlier incident, when she was arrested for allegedly "causing a disturbance" aboard a Texas flight.
Police escorted 27-year-old Blac from a flight following the incident at Austin-Bergstrom International Airport on Friday evening (29 January) when she was removed from the plane.
Blac, whose birth name is Angela Renee White, was arrested for public intoxication and possession of a controlled substance, the Austin Police Department said in a statement to CNN.
The controlled substance was "one gram or more but less than four grams," the statement said.
Blac also took to Instagram sharing a meme which read #FreeChyna.
The arrest comes just days after Blac and Rob seemed to confirm they were dating with a series of social media posts.
The Kardashian family are reportedly unhappy about the union as Blac's previous fiancé Tyga is now dating Kylie Jenner.
Blac and Tyga have a child together, three-year-old son, King Cairo.
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