Great, really great, questions and I hope others will assist us in getting the answers over time. A good book to read on this issue is called, On Being Black and Reformed, (P&R Publishing) by Anthony Carter (a black man). In it, he covers the oppression of the black race, but he also seeks to bring about the beginning of restoration on this issue. There is hope as Carter comments,

Segregation in the church can be analogized to a limp. It is a disability in the body of Christ that everyone sees, that everyone abhors, but that few within the conservative evangelical church have been willing to address. That is, until the glorious reports of recent times. It seems that the evangelical church in America is finally realizing that the limp that is segregation and discrimination in our pews and pulpits is due in large part to white Christians having kicked black Christians at a time in the history of the American church when open arms should have been the order of the day. Nevertheless, the limping is becoming less pronounced.
I believe you are correct on the PCA demographic. Since most are white within the PCA a valuable question is how many black, Muslim, Chinese, Korean (all nations and races should go in this blank _____) are our friends? Though this should be natural, do we make daily strides to cross racial boundaries? Unless we begin making friends across these boundaries then our denomination will never be inclusive of other races though the Kingdom directive is to every nation, and kindred, and tongue, and people (Mat. 28:18-20, Rev. 5:9; 7:9; 14:6, etc.). Yes, we send missionaries to Africa to win and educate the loss, but maybe these same missionaries need to be sent to our hometown churches to win and educate us to unity. However, the conversation must be two-sided and those of other races must be willing to join in or it will never work. They must be willing to forgive and we must all be willing to change.

I do not see where the Reformed doctrines need changing in this regard. Maybe the erroneous interpretation of some bible passages, but not the doctrines themselves. There are organizations already that are Black and Reformed: Black Alliance for Reformed Theology (headed by Anthony Carter) and I am sure there are other organizations attempting strides in these areas. As far as worship there was an article on line called Reformed Theology and African American Presbyterian Worship that is interesting. It speaks about the shaping (praise, prayer and preaching) of worship.

For more on Black Alliance for Reformed Theology (Cyrene Ministries):
In Christ,
5 Solas