Blackett joined Celtic on a season-long loan deal from Premier League outfit United in the summer transfer window.
The youngster, though, was thrown into action after just nine minutes when he came on for Jozo Simunovic.
The England Under-21 international struggled against the Norwegian side and was substituted in the 69th minute.
Former Celtic striker Hartson was following the match, and he was disappointed with Blackett’s performance.
“Against Molde, Blackett looked like a fish out of water,” the former Wales international wrote The Sun. “It was torture to watch and mentally he must be in bits after replacing the again injured Simunovic and then being hooked himself.”
Hartson was impressed with the shift put in against Molde by 18-year-old defender Kieran Tierney.
“Celtic’s defence is a shambles, a total joke,” wrote the former Arsenal star. “It really pains me to see what’s going on.
“I would exclude Kieran Tierney from that. The kid was exceptional at left-back and looks to have a great future. But he needs help from the senior pros beside him.”
Celtic will return to action this afternoon when they take on Ross County away from home at the Global Energy Stadium in the Scottish Premiership.