You have to inherit the gene from both sides as it is recessive.
Both of your parents could be Rh+, yet carry the – recessively and pass it on to you making you Rh-.
It can be written as follows:
Rhesus negative
Rh negative
Rh-
RHD negative
Rh(D) negative
RHD-
Rh(D)-
D negative
D-
… and so on!