Remember that the EPA numbers on the window sticker are NOT the numbers used to calculate the gas tax. The EPA city and highway numbers are run on a dyno without A/C, etc. The dyno numbers are calculated and then three additional tests are performed, one at cold ambient temps, one with A/C, and one with high speeds and acceleration to better capture how vehicles perform in the real world. The dyno numbers are then decremented by the results of the additional three tests for the window sticker.
The gas guzzler tax does NOT include these three additional tests, it just uses the dyno numbers. The real world numbers are typically 70-90% of the dyno numbers for the city rating, and 70-80% for highway. It also has a correction based on number of vehicles produced. If the Blackwing represents a small number of the total fleet vehicles, it may get a small boost reflecting its limited nature.
The CT4 V BW automatic will most likely escape the GGT, the manual is probably right on the border. The CT5 V BW will certainly have a GGT.
For a real world example, Ford was charging a GGT on the Mach 1 with auto tranny, which has a real world fuel economy of 15/18/23: exactly the same as the CT4 V BW manual. At some point, they stopped charging the GGT and refunded the early customers. The manual version had the GGT throughout.