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!
The recessive gene explanation is good. I come from many dominant negative blood relatives and then there are some that are positive blood. And this is the reason that I can see first hand that many of the dominant positive blood relatives have the same traits as all of us dominant negatives. I learned years ago about the dominant and recessive genes and I wish scientist would write more about this. Take for instance one of my siblings is dominant positive and the other siblings are dominant negative. Yet the dominant positive sibling has all the traits that we do. I have always said that this sibling more than likely has the recessive negative gene. For all the millions of people around the world who are dominant positive blood and they post having some of the same traits as many dominant negative blood people. I believe they are carrying the recessive negative gene. When a person gets blood typed they only type the dominant gene. I read years ago, that only certain surgeries such as organ transplants do they need to know both the dominant and recessive gene.
I presented this view when my grandson was born to try and learn more. I was told that my opinion was an outdated Mendelian one, that has been superceded by genomic genetics. I suspect that the two views may not be entirely mutually exclusive, but have had a hard time finding ways to be tested for specific details.
I welcome all well founded and well-informed advice.