According to the rules of the Texas Senate, bills go on the calendar in the order they come out of committee. So no matter what the bill number is, they make sure the blocker bill is the first out of committee.
You raise an interesting point about blocker bills and special sessions, though. As you point out they can only consider the issues which the governor has placed on the agenda for a special session.
Any bill can be introduced and passed in a special session in Texas, however if the bill is outside of the call of the governor any member can kill it just by raising as a point of order that the bill is outside the governor's call. Given the acrimony of the last few months this point may soon be moot, as I don't see what will get the support of all 181 members of both houses.