Don't know of many blacks who joined the Confederates; that's kind of a silly claim. As for why many didn't flee north, that was because they weren't allowed to, and most who tried were confined in 'property camps', where hundreds of thousands of them died, and in any case the Black Codes in most northern states would have made it impossible for them to make a legal living in the north anyway, particularly in Illinois, Lincoln's home state. Add to that the military governors of Union occupied southern states under martial law mostly prevented 'free' blacks from leaving their plantations without written permission from the plantation owners; Lincoln still wanted that cotton picked and the profits pouring into both his Party's pockets and his cronies' pockets, after all. The whole point of his illegal war was loot, after all, not 'freeing the slaves'.