I hope you won't mind a slightly unconventional style of answer...

Seattle remains one of the whitest cities in the US, but it was even more so in the early 20th century, if you can imagine that. The 1900 census recorded 406 black residents. So when we say the African-American population of Seattle increased disproportionately greater than the overall population rise, keep in mind this was not too difficult. In 1900, black Seattlites accounted for 1/2% of the population; 40 years later, this had doubled--doubled!--to 1%.

Seattle wasn't exactly unknown as a destination for black southerners and Midwesterners looking for a new beginning, but its small black community (few people had family support or acquaintance-based job leads) and sheer geographic distance made it few people's first choice. Following four emigrants westward will illustrate this nicely.

Clarice Lawson was born in Arkansas in 1894, but like her sisters, her primary occupation as a teenager was seasonal work picking cotton down in Louisiana. She was raped one season, which was bad enough; and found out she was pregnant, which was even worse. But her sisters rallied and combined forces to send her someplace where she could find better work and hopefully a secure a husband before her pregnancy and future child became an issue (either social stigma or financially). They didn't know where she should go--it was actually on the advice of several kind railroad workers that Lawson ended up in Seattle.

Nora Moore (b. 1883) and Bertran Hendricks (b. 1866) had a different story. They had both made the big move to a northern city already--but that city was Chicago. Moore had moved from Tennessee to pursue a career in vaudeville (she was a talented dancer); Hendricks had just jumped over from Ohio and found work as a stagehand. The two fell in love and eventually married as they traveled the country from vaudeville stage to vaudeville stage. In 1909, their troupe played Seattle and--never left. It was bankrupt a few years later, but Moore and Hendricks decided that the Pacific Northwest offered better opportunities even outside of theatre, and chose to stick around.

Preston Jeter, meanwhile, was a Virginian who had even managed to achieve a college education! ...but still couldn't find work as a black man in the south. He actually didn't plan on Seattle proper: he heard that the mines in Roslyn, a little bit away, were willing to hire black workers in better conditions than his earlier mining experience in West Virginia. It was problems in Roslyn--namely, companies were willing to hire African-American workers to bust white workers' strikes, and race relations (not good to begin with) spiraled downhill from there--that pushed him to close that last gap to Newcastle and then Seattle by 1915.

Seattle's tiny black population, in the face of thriving racism, had elements of blessing but was mostly a curse. The only houses that landlords would (and in some cases, the only ones they legally could) rent to African-Americans were packed in a four square mile block of veritable slums. The tiny black population grouped in an area with other poor emigrants from the east and west along with Native Americans, Esther Mumford Hall argues, led to one of the most tightly knit and multicultural communities in the early 20th century US. Black Seattlites, even without Jim Crow laws (although there were legal housing restrictions in many districts), nurtured community ties further with their own restaurants, social groups, and even a USA-wise famous, majority-black entertainment district known as Jackson Street.

But while Lawson, Moore, Hendricks, and Jeter showed absolutely zero interest in moving back east, they all fell into the grim struggle of trying to make a living in an extremely racist job market. Lawson, like 84% of Seattle's black women residents in the 1910s, worked as a domestic servant. She helped raise white women's children while watching two of her own die and having to put two others up for adoption.

Jeter's pattern of employment was somewhat typical, and typically depressing. Continuing the tradition evident in his mining experience, he and other underemployed African-Americans were essentially scabs for hire. Whenever white workers went on strike, industrialists simply turned to utterly desperate black men. Most of the time, he filled in as a longshoreman or other coastal work.

Moore and Hendricks, the refugees from vaudeville, couldn't even make something like that work out. They actually ended up crossing the border to Vancouver, hoping for better pastures in Canada. This proved not to be the case--Hendricks bounced around from menial job to menial job, the only type open to black men: bathroom attendant, golf course steward.

Their children had trouble, too: one son tried to pick up prize money as a boxer to support himself and his by-then widowed mother, having been rejected from every job for not meeting requirements that mysteriously appeared when prospective employers found out he was black. But while his parents had hoped Vancouver would be better than Seattle, Al worked hard to make the reverse true. He started off bussing tables, then there was shoe shining, and finally, finally--as world war loomed on the horizon--secured an ironworks job that paid a liveable to decent wage. Even that couldn't last, though; he was back racking pool balls when he was drafted into the army.

These four snapshots, I think, displayed the swirl of hopes and dashed hopes that brought African-Americans to Seattle and spun them around in its rich cultural and poor economic environment. I know this is a little different than a typical AskHistorians systematic answer with lists of jobs and community groups, but I mean:

How can you pass up the chance to discuss the grandparents of Jimi Hendrix?

Follow up question: What would be different for an African-American woman?