information:extraction, quality:corruption
Judge

Even after the publication of the Commentaries, Blackstone's chances of judicial appointment remained slim. While he was old enough, experienced enough and widely respected, the presence of Lord Camden as Lord Chancellor and Blackstone's lack of aristocratic patrons at the time hindered his chances. In January 1770, however, Lord Grafton's government began to fall, with Camden resigning on 17 January and Solicitor-General John Dunning, following him. George III appointed Lord North as Prime Minister, and North picked Charles Yorke as Lord Chancellor. Yorke's death on 20 January, after holding the position for less than three days, left several important legal positions within the government open. As such, Blackstone, now MP for Westbury, was apparently approached to become Solicitor-General; he refused, not wanting to deal with the complicated duties attached to the position.

On 9 February 1770 – apparently with the intervention of the King, and possibly Lord Mansfield – Blackstone became a Justice of the Common Pleas, succeeding Edward Clive, and was made a Serjeant-at-Law on 12 February. After only four days it was announced that Joseph Yates was to move to the Common Pleas, and Blackstone was again sworn in as a judge, this time of the Court of King's Bench. This was apparently due to Yates' poor health; Lord Mansfield ran a busy court as Lord Chief Justice, and it was felt that his transfer to the Common Pleas was for the best. Others commented that it was instead due to political and judicial disagreement, with Yates unwilling to stomach the changes which Mansfield made to English law. Blackstone sat regularly as a judge, despite bouts of ill health, and also served on various circuit courts. Prest describes him as an "exceptionally careful, conscientious and well-respected judge... his judgments ranging between narrowly framed technicalities broad statements of public commentary". He was, however, considered a poor trial judge, being reversed on appeal more frequently than any of his peers.

Blackstone returned to the Common Pleas on 25 June 1770, having spent less than six months in the King's Bench; Jeremy Bentham asserted that this was due to Mansfield's having Blackstone removed similarly to his removal of Yates. Bentham asserted that in the King's Bench, Blackstone was "always in hot water", and that there was "heartburning" between the two; Bentham's account is considered dubious because historically, Mansfield and Blackstone had an excellent relationship, with the third volume of the Commentaries describing Mansfield as "a judge, whose masterly acquaintance with the law of nations was known and revered by every state in Europe". There is only one recorded King's Bench case, R v Proprietors of Birmingham Canal Navigation, in which Blackstone and Mansfield disagreed.

In the Common Pleas, Blackstone operated under a civil jurisdiction rather than a mixed civil and criminal one. This played to his strengths, and many of his decisions are considered farsighted; the principle in Blaney v Hendricks, for example, that interest is due on an account where money was lent, which anticipated Section 3 of the Law Reform (Miscellaneous Provisions) Act 1934. Blackstone's decision in Goldswain's Case was later repeated by Lord Denning in Falmouth Boat Construction Co v Howell in 1950.