Blackadder was named in the squad for the 2018 Super Rugby season. He made his debut against the in Round 10 and was later named Crusaders rookie of the year. Blackadder played every minute of the 24–13 win over the in the 2021 Super Rugby Aotearoa final as the side won their fifth title in a row. After an outstanding season Blackadder was voted Champion Crusader of the Year by his teammates at the 2021 Crusaders awards. Blackadder suffered a season ending injury late in the 2022 Super Rugby Pacific season as the Crusaders went on to a 7–21 win over the in the final.
All Blacks
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. He was named in the travelling squad for the 2021 Rugby Championship and the end of year tour of Europe. Blackadder was not selected in the All Blacks World Cup squad for the 2023 competition, however was called in as injury cover.