Unfortunately, the first New York City citation I have is from 1915. I'll post the citations here anyway, hoping eventually to do better.
APPENDED FROM ADDITIONS 1997 block, n. Add: [V.] [23.] block party
U.S., a party, usu. one held outdoors, given by and for all the residents of a
block or neighbourhood.
1919 Red Cross Mag. Oct. 79/2 The *block party idea had just struck Waterbury in the middle of the summer but it seems to have struck hard.
1935 C. F. WARE Greenwich Village 1920-1930 iv. 101 Such general forms of block organization..as the block parties which the political organizations used to stage had
entirely disappeared by 1930.
7 August 1907, Reno (NV) Evening Gazette, pg. 2?, col. 1:
PHILADELPHIA, August 6. - Two good-looking young men were the star attractions at a block party for the North American Outing Fund a few night ago, in Smedley street, between Westmoreland and Ontario streets. They announced that they were prepared to sell kisses for 5 cents each.
14 August 1908, Atlanta Constitution, pg. 7, col. 2:
"BLOCK PARTY" TONIGHT
AT ST. NICHOLAS RINK
Those who have been wondering what a "Block Party" is like can find out tonight by going to the St. Nicholas Auditorium; and, judging from the many expressions of interest and curiousity heard for the past few days, there will be a very large attendance.
A fine prize is to be awarded to the skating couple who are clever or fortunate enough to evade the elimination rules until all other couples have been retired from the floor.
22 September 1908, Frederick (MD) News, pg. 5:
Skating and dancing at Lake View Casino, Wednesday night. Something new for skaters, block party, very interesting. Prize to winners. Try for the prize.
11 March 1914, Chicago Daily Tribune, pg. 5:
Starts "Block Parties."
Miss Harriet Vittum, independent candidate for alderman for the Seventeenth ward, has started a series of "block parties." Every woman in the block is urged to attend the party held in her block.
In the afternoon a Yiddish block party was held. Tomorow night there will be a Scandinavian "block party" in the home of one of Miss Vittum's friends. From two to four of them a day are planned for the next few weeks.
6 July 1915., New York Times, pg. 18:
"Block parties" and other neighborhood celebrations were held in many parts of the borough.