We are being pushed to buy oil from a blender that is not API certified. How concerned should I be aout this?

I would recommend that you dont. try and find a first rate manufacturer that will supply you direct dont be shy visit there plant check for quality control try to talk to some of their customers.

I am new to the lubrication field, but I have found the difference between the performance of many "good" and "excellent" oils is profound. If you do not care about how long your equipment lasts, use anything out there. If you want to improve your efficiency, create less heat due to lowered friction coefficients, use less energy, save money over the long haul, use the best you can find.
Have the rep test his product along with all the rest of the excellent products, in front of you. You should be able to see the difference.
This is how I sell my product. Observable results is the important aspect to overcome.