Why does ac only blow hot air with the ac compressor working in a 2007 grand marquis?

Could be a number of reasons. There may be blockage which prevents air from flowing over the fins of the condenser. There may be a blockage elsewhere in the AC system. It could be insufficient refrigerant.

You can try cleaning off your condenser (I would not recommend using a pressure washer for this, whatever you do - condenser fins are brittle and bend easily). If that doesn't rectify the problem, you'll need to take it into a shop and have a system performance test conducted.