What if someone is a victim of blackmail and is forced to commit a crime?
The offence of blackmail is contrary to section1, Theft Act 1968 and is, succinctly, making unwarranted demands with menaces (i.e. threats).
Although there is an "act", one criterion for actus reus is for it to be a voluntary act which, if the threats are sufficiently menacing, may not be so.
Depending on what was the blackmailed threat was, it is possible to claim the common law defence of duress. This, as per this article by the Crown Prosecution Service:
is a defence at common law to all crimes except murder, attempted murder and treason involving the death of the sovereign see R v Gotts  2 AC 412.
To successfully apply this defence the threats have to be threats of death or grievous bodily harm toward the blackmailed person or a member of their immediate family or to a person for whose safety the they would reasonably regard himself as responsible. (There is case law on this, but I've yet to track down a suitable report or summary.)
Also, the threat has to be an effective threat at the time the offence was committed. See R v Hammond  EWCA Crim 2709 where the Court of Appeal ruled that defence of duress was correctly withdrawn from the jury at trial because the evidence could not satisfy the requirement that the threat must be imminent or immediate and have been operating on the actions which constitute the criminal conduct.