There's a paper on them:
The Waltham Blacks and the Black Act
Pat Rogers
The Historical Journal, Vol. 17, No. 3. (Sep., 1974), pp. 465-486.
http://links.jstor.org/sici?sici=0018-246X%28197409%2917%3A3%3C465%3ATWBATB%3E2.0.CO%3B2-8 - if you can't access this link it's because you probably need to be on an IP with access (university or library possibly).
In summary: this paper refers to Mr Shorter's group as the Berkshire Blacks, and he was a farmer said to rent an estate of £200 a year and worth £80 of his own. Their principle target was first Earl Cadogan's estate at Caversham Park. Late in April 1723, the government took action and managed to bring to London three of the leaders by subterfuge around May 1st, intending to interrogate them - but William Shorter, described as 'King John of the Blacks', escaped by breaking through the wall of the room he was held in (it also mentions here that William Shorter's son, also William, was also a Black, but not an important one). The other two, George Wynne, a clockmaker and Edward Collier, feltmaker, were held and further members were questioned, some of whom gave evidence against a number of others. 32 Blacks were tried at Reading: a caravan of 23 prisoners arrived on 3 Jun followed by 9 more two days later. Four were convicted of murder for a killing at Old Windsor (Thorne, Hawthorne, Gilbert and Hatch), and were executed of 15th June. Three were transported for deer stealing and assault, three more for deer stealing only (although two of these gained a respite).
One of these men was Robert Shorter, who died in prison sometime before 17th June.
William Shorter was still at large, and this article does not mention him ever being brought up for trial or hanging; perhaps he had enough influence to avoid having anything pinned on him.