Critical reception to Biffy Clyro's debut album was generally mixed. John Murphy of independent music website musicOMH, writing at the time of its release in 2002, claimed that Blackened Sky was evidence of Biffy's status as "the most exciting new band in Britain", drawing comparisons to influencing band Nirvana and praising elements such as the consistency in the vocal performances. His review was not entirely positive though, as he pointed out that "with this album ... the band sometimes lose the balance between light and shade", claiming that "There’s no sign of any kind of breakthrough song here, ... but if you take the time to explore this Blackened Sky you’ll find a few shining stars worth exploring". Jason MacNeil of PopMatters was similarly indecisive, concluding that Blackened Sky was "Generally a credible first step despite some lapses in judgement".
Allmusic critic Dean Carlson was less generous, awarding the album a low rating of 1.5 out of five stars; the review proposed that on the album "the band came across as all of the least interesting aspects of American grunge released a decade too late", with the website claiming that Biffy did not live up to the hype they had received prior to the album's release.