They were treated quite well. Since fugitive slave laws were still in place in the United States until 1862 when the Act Prohibiting the Return of Slaves was passed which meant that any slave which was in a territory controlled by Northern troops was ipso facto free since they were really seen as contraband, essentially captured enemy property.

Canada was where around 75% of all escaped slaves ended up, around 100,000 in total who mostly settled in Ontario since after all, it was the closest. In terms of if they were welcomed, they certainly were. A special order-in-council which received Royal Consent in 1855 which made it an official policy to accept runaway slaves and provide food, clothing and shelter to new arrivals, as well to arrest and imprison the bounty hunters/kidnappers sent from the slaves masters to return their property, were arrested at the border. As more and more waves came to Canada and began to adapt the environment (a stark contrast to the South) small towns of entirely escaped slaves began to form around the St.Lawrence and York (Toronto today) and Amhurstburg where some grew tobacco.

Although the government and many citizens were tolerable and accepting of the newcomers, especially early on around 1830. But in 1850 as more and more came more citizens began to harbour racist views. Turing a blind eye to their struggle during their establishment and there were few cases of assaults and murdered against black immigrants. But you do have to consider that there is a potential number of cases which were never recorded or reported due to these racist views, however there is no evidence of this. Institutionalized racism also meant it was harder for black immigrants to find work, government support or schooling.

However they did not tolerate this, after the Civil War in 1870 many began to expand into the West. Tired of the racism in the East they joined with many European immigrants moving into the praries to establish thriving cities like Calgary, Vancouver and Winnipeg. They were even invited by James Douglas (Governor of BC) to Victoria, where the first black politician held office (1871). As time went on many more Black immigrants became more established across the country and the abolition movement grew, Toronto even hosted the North American Convention of Colored Freeman.

And something to remember is that the Emancipation Proclamation did not free slaves throughout the US. It only applied to states which seceded from the Union which left slavery untouched in the South and loyal border states. So many were not quick to return to the US as it a) cost money, most of which was spent getting to Canada b) is still dangerous and the war is still alive and c) emancipation was contingent on Union military victory, which would not offically occur until 1865, two years after the Emancipation Proclaimation.

Hope this helps :)

You may be interested in parts of A North-Side View of Slavery by Benjamin Drew; he went around to several cities and towns in Canada in the mid-1850s and interviewed fugitive slaves living in them. The accounts are mostly about their experiences in slavery, but his introductory sections give general overviews of the black population in each town, and in some cases the introduction or one of the narratives talks about some major issue they were dealing with. (So for example, William Jackson in Queen's Bush talks about problems people there were having with paying for land they had cleared.) Another common thing that comes up is schooling, which I think was available to everyone, but integrated in some places and segregated in others. They did also face prejudice in Canada; for example, the section on London mentions resistance to black students in the (integrated) schools, and a few of the narratives in that section talk about prejudice, though they generally didn't find it as terrible as in the US.