* Wheat at new 21-month low, down for seventh session

* Plains rain, global supplies outweigh Black Sea deal risks

* China cancels sales of 233,000 tonnes of U.S. old-crop corn

MEXICO CITY, April 27 (Reuters) - Chicago wheat futures fell for a seventh straight session on Thursday and touched a 21-month low on export competition while corn hit a nine-month low, walloped by cancellations of U.S. corn sales to China.

The most-active wheat contract on the Chicago Board of Trade (CBOT) dropped 12-3/4 cents to settle at $6.29-1/4 a bushel, after falling to $6.25-1/2, the lowest on a continuous chart since July 2021.

CBOT corn slipped more than 3%, settling down 19-1/2 cents at $5.81-1/2 a bushel after dipping to $5.80-1/2, the lowest since July 2022.

Corn declined after the U.S. Department of Agriculture said private exporters cancelled sales of 233,000 tonnes of U.S. old-crop corn to China, underscoring concerns that a large Brazilian crop was diverting demand from the United States.

"We're unwinding a lot of the bullish news. Russia has a large wheat crop they are selling into the world market, Ukraine is trying to sell their grain into the world market, and the EU is trying to keep grain from coming into their countries," said Don Roose, president of Iowa-based U.S. Commodities.

Rain this week in the southern U.S. Plains has tempered concerns about drought regarding hard red winter wheat.

Global wheat supplies have also led the market to look beyond repeated warnings from Moscow that it will pull out of a Black Sea grain deal allowing shipments from Ukraine unless Russian demands are addressed.

Soybeans settled down 11 cents at $14.03-3/4 a bushel.

Advancing U.S. spring planting and competition from Brazil were a drag on Chicago corn and soybeans.

"There is too much competition on corn and soybeans, and the weather is open for planting," Roose added.

Net sales of U.S. corn for shipment in the current and upcoming marketing years fell to a 15-week low of 400,000 tonnes in the week ended April 20, USDA data showed. (Reporting by Cassandra Garrison in Mexico City, Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Kirsten Donovan and Jonathan Oatis)