How do I find out about the pitch/gradient of the different blue runs at Park City. Some blue runs are very much more difficult than others

Ask the ski patrol or the mountain guides with the well marked park city jackets, they hang out by the trail maps at the top of the main lifts. They can help you decide on the run more than knowing the gradient.

Typically the chairs will tell you what types of runs they service you up to. It also has the vertical feet the chair brings you up to. Not sure about the pitch though I'll try and figure that out for you.