Dear "CK", That's right, in KIDNEY transplants the "+" or "-" does not matter.
Also, be aware that for kidney donation the blood types do not always have to be IDENTICAL. They have to be COMPATIBLE. For example, a type "A" recipient can receive a kidney from another type "A" , but also from a type "O". However a type "O" can only receive from another type "O". A type "AB" can receive from any type.
best wishes,
Fr. Pat