When a new black hole is created I will describe its effects (for instance if Marseilles were black holed in the coming Spring, the effects would be that the unit in Marseilles at the end of that turn is destroyed, Burgundy gets a coast, Gascogne gets two coasts, and since there would now be 33 SCs left the victory condition becomes control of 17 SCs instead of 18). Rules are very simple (taken from 'Tinamou', David Partridge's subzine in Jamie McQuinn's 'Crossing the Rubicon'):
(1) After each Spring and Fall season, a black hole will come into being. A space that has been black holed becomes, in effect, a thick border. All spaces that were adjacent to the space before it became a black hole are now adjacent to each other. Any space on the board, with the exceptions noted below, may be black holed.
(2) Retreat orders are submitted separately. However, retreats are considered to have occurred prior to the creation of the black hole. Thus the retreat is made with the foreknowledge of where the black hole will go. Therefore, a unit forced to retreat from a space that is black holed is not destroyed, but may retreat. A retreating unit may NOT make use of the new black hole as it has not yet been placed. If the only legal space to retreat to is black holed, then the retreating unit is destroyed.
(3) Each power designates one of its home centres as immune to black holes. This designation is secret and known only to the player and the GM. Don't forget this!
(4) Switzerland may not be black holed.