In a long article about 80s metal, Tim Holmes of Rolling Stone wrote a contemporary review about Twisted Sister describing them as "the clown heir apparent to the gaping vacancy left by Alice Cooper" and a band who "write(s) songs that have a giddy, street-smart narrative approach and a gritty coherence that metal usually lacks." He also wrote that Under the Blade "is not technically a new album but rather a remix for modern ears" of older music.
Modern reviews are very positive. Greg Prato of AllMusic reminds how the band moved to the UK, which was having a "heavy metal resurgence (dubbed the New Wave of British Heavy Metal)", to record with UFO bassist Pete Way "many of the band's best compositions from their club days" and finds Under the Blade "one of Twisted Sister's hardest rocking albums... highly recommended to lovers of early-'80s British heavy metal." Also Exclaim! reviewer Ian Gormely considers the album "a must for anyone with an interest in the history of American hard rock". Despite "the raw production... and lack of an obvious hit... it laid the groundwork for their future success", thanks also to Twisted Sister's "tongue-in-cheek presentation that latter-day hair metal bands clearly lacked." Adrian Begrand of PopMatters reviews the album as a "near-classic" and "the most ferocious of the band’s career." He writes that the musicians may have "bar band roots", but on the album "the fun side of Twisted Sister is set aside in favor of something a lot darker", which brought to "a hell of a debut that not only connected with British heavy metal fans, but would eventually lead to a contract with Atlantic Records, paving the way to stardom a couple years later." Canadian journalist Martin Popoff considers Under the Blade "dead serious despite the garish imagery, a good four-fifths of it rocking with hellacious clout, attitude and clever economy" and remarks how the influence of Judas Priest is evident in Dee Snider's compositions.
In 2005, Under the Blade was ranked number 387 in Rock Hard magazine's book of The 500 Greatest Rock & Metal Albums of All Time.