“We find, in the rules laid down by the greatest English Judges, who have been the brightest of mankind; We are to look upon it as more beneficial, that many guilty persons should escape unpunished, than one innocent person should suffer. The reason is, because it’s of more importance to community, that innocence should be protected, than it is, that guilt should be punished; for guilt and crimes are so frequent in the world, that all of them cannot be punished; and many times they happen in such a manner, that it is not of much consequence to the public, whether they are punished or not. But when innocence itself, is brought to the bar and condemned, especially to die, the subject will exclaim, it is immaterial to me, whether I behave well or ill; for virtue itself, is no security. And if such a sentiment as this, should take place in the mind of the subject, there would be an end to all security what so ever.”

De laudibus legum Angliae (c. 1470), reported in Bartlett's Familiar Quotations, 10th ed. (1919).

Attributed by Edward Seymour in 1696 during the parliamentary proceedings against John Fenwick ( "I am of the same opinion with the Roman, who, in the case of Catiline, declared, he had rather ten guilty persons should escape, than one innocent should suffer" http://books.google.com/books?id=dIM-AQAAMAAJ&pg=PA565), to which Lieutenant General Harry Mordaunt replied "The worthy member who spoke last seems to have forgot, that the Roman who made that declaration was suspected of being a conspirator himself" (Caesar was the only one who spoke in the Senate against executing Catiline's co-conspirators and was indeed suspected by some to be involved in the plot). However, the Caesar's corresponding speech as transmitted by Sallust http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Sallust/Bellum_Catilinae*.html#51 contains no such phrase, even though it appears to be somewhat similar in spirit ("Whatever befalls these prisoners will be well deserved; but you, Fathers of the Senate, are called upon to consider how your action will affect other criminals. All bad precedents have originated in cases which were good; but when the control of the government falls into the hands of men who are incompetent or bad, your new precedent is transferred from those who well deserve and merit such punishment to the undeserving and blameless.") The first person to undoubtedly utter such a dictum was in fact John Fortescue ("It is better to allow twenty criminals to mercifully avoid death than to unjustly condemn one innocent person"). It should also be noted that whether the exchange between Seymour and Mordaunt even happened is itself not clearly established http://books.google.com/books?id=IitDAAAAcAAJ&pg=PA694.
Misattributed

Victor Frankenstein of Justine Moritz in Ch. 8
Frankenstein (1818)

Letter to Benjamin Vaughan https://books.google.de/books?id=d3UPAAAAQAAJ&pg=PA166&dq=maxim, on Blackstone's Ratio (14 March 1785).
Epistles

On Crimes and Punishments (1764)

The Power and the Glory (1940)

Cases of Conscience Concerning Evil Spirits Personating Men, Witchcrafts, infallible Proofs of Guilt in such as are accused with that Crime (1692); a variant of this has become known as Blackstone's formulation, through its expression by William Blackstone in Commentaries on the Laws of England (1765 - 1769).

As quoted in "Justice under Reagan: Reagan seeks judges with 'traditional approach" (14 October 1985), U.S. News & World Report, p. 67
1980s