Perhaps unsurprisingly, hockey was the last of the four major sports in North America to break the colour barrier, doing so in 1958. However it should be noted that this was not due to any unofficial ban on black players, and indeed the first minority had played in the NHL in 1948 when Larry Kwong, who was of Chinese descent, played one game for the New York Rangers. But it was Willie O'Ree who was the first black NHLer, who made his debut for the Boston Bruins on January 18, 1958.

O'Ree was a star player in the Quebec Hockey League, a minor league that was of a fairly high calibre (and good pay in comparison with the NHL of the day). He was also more notable not for being black, but for the fact that he was nearly blind in one eye, the result of a puck to the face during a game several years before; such an injury should have kept him from making the NHL, but he managed to cheat on his eye exams. His call up only lasted for two games in 1958, and a further 43 during 1960-61, and he was not close to a star in the NHL. It would also be 14 years until the second black player made the NHL when Mike Marson played for the expansion Washington Capitals in 1974-75. In terms of racism faced, O'Ree noted it did happen, but was more seen in the US than Canada, with references to going back to the fields and the like.

I will also note that O'Ree should not have been the first black player in the NHL. Throughout the 1940s and 1950s one of the top players in the Quebec League was Herb Carnegie. He was offered a tryout and contract by the New York Rangers in 1948, but turned it down due to the low pay compared to what he was making in Quebec. There is also the unfounded rumour that around this time Conn Smythe, the owner and general manager of the Toronto Maple Leafs, said he's "pay $10,000 to anyone who could turn Herb Carnegie white," a reference to the fact that Carnegie was good but the prevailing racial sentiment meant it would be impossible to have him on the team (whether this was actually said by Smythe or not is immaterial, as the sentiment is true). Carnegie would finish out his career in the Quebec League, a well-regarded star in the league.

Overall though, I'd say that there were (and still are) two major factors for black hockey players. One, the sport is mainly played by Canadians, and the population of black Canadians is relatively low, so it was going to be a struggle to have many black hockey players to begin with. Secondly, the cost factor in playing hockey is huge. In both Canada and the US, blacks are on the whole of a lower socio-economic status, and hockey has steadily become a upper-middle class sport, with fees and infrastructure far higher than the other three sports in North America. This precludes many lower-income participants, which has a disproportionate effect on the black community. The side consequence of that is there are fewer black players, and thus fewer black star players, to inspire youth to take up the game. However that is slowly changing, with several notable black players in recent years (Jarome Iginla, now PK Subban, to name the two biggest ones) reaching superstar levels. But so long as it remains primarily a sport for wealthier Canadians, the number of blacks in the NHL will remain low.