Snake brought an edge case to my attention over in the MX Discord, so I thought I'd bring my verdict here and provide some examples. The edge essentially was where an obstacle on the road forces the vehicle to go in the forbidden direction without changing the blockline. Here's some examples, we'll assume that the start faces north.
Above, you can see that the northern half of one of the blocks is blocked off, forcing the vehicle to drive to the south of it. This is acceptable; the turn before the obstacle allows the vehicle to stay in the southern half of the line, thus meaning that it's possible to take a line that never turns south. Even if the course's fastest line goes into the northern half of the blockline before turning south to get around the obstacle, this would still be legal! There would only be a problem if the fastest line did not remain in the shown blockline but went north of that before returning south.
In this case, now there is an obstacle which forces your car to go into the northern half of the blockline before going into the southern half. Because the route now requires you to drive south, it is no longer legal. Despite this, I won't be disqualifying any maps which break this rule; it's all in the same blockline, so it's a rather minor infraction. Infractions like this will cause losses in the "theme" category of judging, though, so be careful.