What if any is the difference between a Black top A series and a blue top?

Black tops are the later model A15 import engines. Depending on market/year may have smog heads and electronic ignition.

Thanks

Any power differences?

I currently have a blue top A15. I have been offered a Black top A14.

Which of the two would be the prefered?

The Blue Top A15 would be the best option is if it is in the same condition as the A14. A15 will produce more torque - hp about the same but future hp potential more from A15.

A14 often have better availability, meaning you can find one at an inexpensive price.
A15 is 7% larger, meaning an automatic power increase in low and mid-range RPMs, all else being equal

A14 and A15 stock bottom ends can both rev to 8,500 rpm so there's no advantage there.
If both use the same cylinder head, there is little top-end power advantage with either engine.

Thanks