blackwood started his career with petershill and joined scottish first division club celtic on 23 september 1899. he and willie mcoustra were signed by the club after playing in a friendly against the kaffirs, a touring team from the orange free state. after a single appearance, he was loaned to partick thistle of the second division the following month. he made his debut for thistle in november, replacing willie paul in the forward line in a 2–0 win over airdrieonians, and scored his first goals later in the month, against morton. in december, he scored twice, the first in the opening minute, in an 8–1 win against linthouse. he continued to score goals for thistle into the new year, with eight goals in two friendlies against clachnacuddin and orion, and a hat-trick against st mirren in the western league. he had been due to play in the scottish cup first round match against galston on 13 january 1900. the club had arranged with the caledonian railway for the train carrying the team to stop at gorbals to pick up blackwood and william goudie, but it did not stop. he did feature twice in the competition, scoring once, and scored seven goals in eight appearances for thistle in the league. at the end of the season, blackwood was recalled by celtic, who then sold him to football league second division club woolwich arsenal.