A rainy spell, usu in spring or early summer and associated with the flowering of wild blackberry bushes, but occas later in the year and associated with the ripening of blackberries.
1836 in 1950 Clopper Amer. Family 235, Again in June of 1836 she wrote four stanzas extempore on seeing Mary Ann “gazing on two stars and admiring the firefly during the blackberry rain.” 1867 (1871) Garrett Excelsior Dialogues 182, Yes—since the blackberry rain the crops have had a smart chance to ripen. [DARE Ed: The speaker of this line is identified as “a southerner.”] 1884 IL Daily Courier (Jacksonville) 22 May 3/1, We have passed through the “seven wet weeks,” the “whippoorwill storm,” the “dry, cold moon,” and now we are having the “blackberry rain.” 1894 News (Frederick MD) 25 July /1 (newspaperarchive.com), Now that we have had the blackberry rain, housekeepers will soon be making jam. 1899 Bergen Animal Lore 108 MD, There is a rain at the time of the dogwood (Cornus florida) blooming, hence called “dogwood rain,” and another when blackberries are getting ripe, hence called “blackberry rain.” Chestertown, Md. 1913 Kokomo Daily Tribune (IN) 13 May 4/5, It is going to rain May 20. On that day Indiana has what we old-timers have always called “the blackberry rain.” 1916 Chester Times (PA) 8 June 3/3, Yesterday’s rain was the beginning of the Blackberry rain which usually continues three days. 1947 Sun (Baltimore MD) 27 May 18/3 (Hench Coll.), The rains of the past week in Maryland were of the kind we used to refer to as the blackberry rains; and, sure enough, in keeping with this tradition, the blackberry bloom all over the State has come forth. 1958 Press–Gaz. (Hillsboro OH) 17 June 2/2, Blackberry Rain—We’re having a very cold rain as this is written. It’s what some farmers call a sheep-killing rain in May and it’s just that. If the newly-shorn ewes and lambs get wet and chilled they are sure to get colds and you may lose some of them.