Green’s Dictionary of Slang

1. a small quantity over and above the stated measure, given to a customer by a stall-holder or shopkeeper.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Blessing a small quantity over & above the Measure given by Dealers in Pease, Beans & other Vegetables.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: blessing. A small quantity over and above the measure, usually given by hucksters dealing in peas, beans, and other vegetables.
1811		Lex. Balatronicum.
1867		J. Greenwood Unsentimental Journeys 123: The ‘blessing’ is a few cresses thrown in over and above the measured quantity, and evidently a ‘hand’ bargain was never completed without it.

2. (Irish) a tip, a handout.

1909

J. Ware Passing Eng. of the Victorian Era 33/2: Blessing (Irish) Gratuity [...] ‘Sure, he’s a man gives me a blessing every time he passes.’.