A ) Clearly, This is not encoder strip problem at all.
Encoder strip only task is to provide metric of the linear distance when head is moving back and forth. And it is independent from any of the color only the print head carriage and to the MB.
B ) If you did changed the new black print head with the new one, and still having this problem; it look like a black print resistor or capacitor on your circuit board is blown. So the black ink is the only one having this reluctant to print. This won't be easy to fix, first you need a T3 level tech to probe your circuit board, and most of the time, just replacement of circuit board would be easier.
Circuit board that driving black color may be an integration to the main motherboard. If that is the case, then you may not have to replace entire MB. You need a certified tech to at least tell you what really went wrong, and how they can fix it. If you connect the good head to a bad circuit, it can actually ruin your good new print head also!