The son of former All Blacks captain Todd Blackadder, Ethan was born in Rangiora and attended Nelson College in New Zealand.

Blackadder was called into the Tasman provincial team in 2016 after a stellar club season.

He became a regular starter for Tasman in 2017 as they made their way to the Premiership Final and in 2019, Blackadder helped the side secure its first provincial title.

With the ability to play both lock and in the loose forwards, the versatile player made his Crusaders debut in 2018 and quickly became a key member of the side. After an outstanding 2021 Super Rugby season Blackadder was named in the All Blacks squad to play Tonga and Fiji in the July Steinlager Series. He made his debut against Tonga at Mount Smart Stadium, coming off the bench in a 102–0 win for the All Blacks - becoming All Black number 1195.