An enclave is a territory, or a part of a territory, that is entirely surrounded by the territory of one other state.
An exclave is a portion of a state or territory geographically separated from the main part by surrounding alien territory (of one or more states).
Semi-enclaves and semi-exclaves are areas that, except for possessing an unsurrounded sea border, would otherwise be enclaves or exclaves.
On this map, after secession, Miami, Illinois, and Cleveland would become both semi-enclaves and semi-exclaves. Detroit and Duluth (Minnesota) would not if they became provinces of Canada. Miami, Illinois, and Cleveland would be semi-exclaves because the land routes to each other would only be through a foreign country. But they would not be full exclaves because they can communicate via international waters. They would be semi-enclaves because they would be surrounded by a foreign country, except by water. The waters are what prevent them from being full enclaves.
List of areas that are connected to the mother country by a land route through another country rather than being attached by land to the mother country (cf. pene-exclave):
Point Roberts, Washington State, USA
The Northwest Angle, Minnesota, USA
West Berlin 1945-1989.
Oecusse, East Timor
List of areas that are connected to the mother country only by water with no land route:
Hawaii, and all US Territories, and any island that is not a nation.
List of areas that are connected to the mother country only by water but border on other countries:
French Guiana (legally an integral part of France, as French as any province)
Spanish semi-enclaves at Morocco
Rio Muni (Equatorial Guinea)
Enclaved by a single nation: Lesotho, San Marino, Vatican City
As for access to each blue state by blue state troops, they could fly in or take a ship, as the Great Lakes are international waters with access to the Atlantic via the St Lawrence seaway.
Also remember Minnesota and Detroit can get to New York via Ontario if they don’t want to go through red states.
But obviously there has to be an agreement with the red states on free passage of goods and people or this whole thing will be cumbersome. Fortunately the blue states have leverage over the red states because the blue states have most of the ports.
Also, different groups of blue states could become independent countries, especially if they decided to join a North American Union (NAU) with Canada and probably Mexico. In the world today, the average nation (not including India and China) has 23 million people, and the median nation has 9 million people. The average US state has 6.6 million.
If they do join a NAU, then free passage can occur through Canada and Mexico too.