There are some general accounts of how and when Scots came to Ulster in the 1600s. If you would like a detailed description, I can recommend Philip Robinson’s “The Plantation of Ulster”. It covers 1600 – 1670.
According to that volume (pages 115 & 175) the main ports in use then by the settlers were Carlingford, Strangford, Donaghadee, Bangor, Carrickfergus, Coleraine and Londonderry, though there were various “creeks” that were used too. Belfast was not a significant port in the early 1600s (Carrickfergus was still the principal town and port in Ulster then) and Belfast was just a muddy river crossing with a small population and no decent harbour.
There were many complaints about Scottish pedlars coming to Ireland in the 1600s and using unguarded creeks to sell smuggled goods and to export stolen corn.
There was a big wave of Scots migration in the period 1610-1625 which was the main Plantation. Then in the 1640s a 10,000 strong Scottish army was disbanded at Carrickfergus (after the 1641 uprising). Many of them chose to stay in Ireland. Many Scots fled Ireland in the 1640s, and went back to Scotland, due to the uprising but then returned when things had calmed down. There was also a huge wave of migration in the 1690s due to famine in Scotland. Some estimates are that 200,000 Scots settled in Ireland in the 1600s. That would be about 20% of the entire Scottish population at the time.
I looked in the Muster Rolls for Co Armagh (c 1630). Edited by R.J. Hunter. There were no Blairs listed then at all, so I’d say that your ancestors arrived post 1630.
During the main Plantation period, some tenants accompanied their landlords in Scotland and so you can see where they came from, from that. (However there was internal migration, and later waves of migrants, all of which muddy the water somewhat).