If you look at a map you will see that the Crete-Libyia gap is less than 200 miles wide. At 250 the tankers would have been vulnerable had they flown south of the FIR boundary and probably upset the Greeks if they flew to the north.
I would guess the black line was the FIR boundary.
Although the airspace between the territorial limit and the FIR boundary was 'open FIR' and not under national control the Greeks did not see it that way - much as we police NATO airspace around UK the Greeks policed their FIR.
Unless we had formal diplomatic clearance we often had to transit on the FIR boundary.