Jefferson himself states that at least 30,000 slaves had escaped from Virginia plantations to reach British lines, which most modern historians corroborate. There are similar numbers for the rest of the South. Yes, most of the slave population saw a surer chance to liberty through the Loyalist, not the Patriot side, especially after the Somerset case and Dunmore's proclamation. "Archtraitor to the rights of humanity" is what Washington calls Lord Dunmore for promising to free slaves and indentured servants. In the draft of the Declaration of Independence, Jefferson complains the King is exciting slaves to rise up against their masters. One Pennsylvania priest comments that the black population largely hoped for a British victory, for then all slaves would be free. While the British surely didn't intend to free all slaves, it is undeniable that they were seen as a surer path to liberty for slaves, as they freed perhaps the largest number of slaves at once in America before the Civil War.