BLI is by far the closest airport to Blaine and would allow you to skip the Seattle traffic altogether.
To get to BLI you can go DAY-DTW-SEA-BLI-SEA-DTW-DAY for $307 (5854 miles 5.2cpm). Sample dates: Thu Oct 25, return Oct 26-27.
(DAY-MSP-SEA-BLI although a few less miles is more widely available.)
For a mileage run to SJU you can also go from DAY:
DAY-MSP-JFK-SJU-JFK-MSP-DAY for $247 (6400 miles 3.8cpm). Sample dates: Oct 30-31. If you want to stay longer there are many other dates available for $10 more.