Michael McLeod had a goal and an assist, and Janne Kuokkanen had two assists for the Devils, who are 4-0-0 in the preseason.
Blackwood has allowed one goal in each of his past two games; he made 15 saves in a 2-1 overtime win against the New York Islanders in Bridgeport, Connecticut, on Saturday.
Connor McMichael scored for the Capitals, who have lost their first four games of the preseason (0-3-1). Vitek Vanecek made 22 saves.
Washington defenseman Martin Fehervary left the game in the second period because of an upper-body injury.
