I’ve personally never worked with either. My brother worked with CRST yers ago. If you are going to have to choose one go with Blair. CRST has a lot of cheap freight just like you’ll find running the spot market. Blair is an arm of P&S Logistics. They are in every place that I haul. From steel plants to lumber yards. So your load choice will be better.
I just returned my equipment to Blair. I ran flatbed with Prime previous to that. I was much more successful with Prime. Unfortunately, I had to stay out 6 weeks roughly stay in the money.
I went to Blair with the hopes of making the same money but getting home more. I got home but the money was not there. The trucks are ex P&S trucks and frankly they want too much money for them. Their freight network is too small for the amount of trucks so you will be getting many loads from spot/ch robinson.
If you live in the Birmingham area it could be worthwhile as many of their customer accounts are located there, but otherwise I would advise to avoid them.
Hope that helps.