David William Blair joined Edinburgh Rugby Academy for season 2007-2008. In doing so, he linked up at Edinburgh with his elder brother, Mike, the Scotland scrum half. At the end of that season David was in the Scotland A squad who visited North America for the Barclays Churchill Cup. He played in the two wins there, as a replacement against Canada in Ottawa and as starting stand-off against Argentina in Kingston, Ontario. He gained a full-time contract in summer 2008, and a further Scotland A appearance followed as a replacement in the 22-all draw with Italy A at McDiarmid Park, Perth, in February 2009.

He made two more Scotland A appearances in the IRB Nations Cup tournament in Bucharest later that year, kicking five goals (12 points) in the 27-3 win against Uruguay. He went on to feature in the 2010 tournament, playing against Georgia, Namibia and Argentina Jaguars, having also seen service against Italy A earlier in the year. That November, he started against USA Eagles at Netherdale, scoring eight points in a 25-0 win, and went on to star in the win over Ireland Wolfhounds in January 2011 and the 11-3 success against Italy A the following month.

David joined Sale Sharks as an academy player after leaving Edinburgh Academy in 2003 and joined their senior squad in 2006. He made his debut for the Guinness Premiership side in 2004 off the bench against Leicester Tigers and made his first start against Gloucester in 2007. He is only the player to score 100 points for Scotland in under-21 internationals, a target he achieved with his second penalty against Italy in Benevento in March 2006: he raised his tally to 102 points by converting Steve Biggart's late try for a 21-14 victory in that match. His record extended to 135 points three months later in the IRB Under 21 World Championship in France. David, who dropped a goal on his under-21 international debut against France in Bourgoin in February 2005, scored 88 points over the 2005 and 2006 under-21 six nations campaigns. His 43 points in 2005 included 18 (with a try) against Italy in Inverness, his overall performance there earning him the man-of-the-match award, and then 14 in the 19-17 win against England at Northampton. Because of injury his participation on the subsequent under-21 world championship in Argentina was limited to two matches, but he returned to the national under-21 team as captain throughout the shadow six nations championship in 2006, including wins in away matches against Ireland and Italy. He scored 19 points, including a try, in the 24-21 victory against the Irish in Athlone. He was also captain for the under-21 world championship.

David has represented Edinburgh at under-16, under-18 and under-19 level and Scotland at under-18 and under-19 as well as under-21. His international honours at under-19 included four games in the age group's IRB world championship in Durban in 2004, when he scored 14 points, with a try against Wales.