The comments section on Cross-Currents is interesting; while my own rabbeim decried racism in all forms, and I recall one being shocked by one student's racist comments almost to the point of anger at the very idea, I've heard way too many racist comments from individuals and even rabbeim in the Orthodox community. I don't think that most are racist, but to say a sizable minority are - particularly in the more yeshivish community - is not a stretch. It's hard to gauge if those percentages are much larger than the world at large, but my own guess would be that it is - though it's possible that because people feel they're in a "safe", closed environment they are simply more prone to expressing that which others might keep to themselves.
In particular, your friends’ warnings that the Orthodox “are rigid and racist” caught my eye. You repeat it twice, but then go on to recount your personal experience as you began dating: “Orthodox rabbis and congregants were veritably welcoming, with one prominent Orthodox rabbi promising to find me a wife as he encouraged me to move into his Brooklyn community.”
My expectation, at that point, was that you would reflect at least momentarily upon the disparity between the misinformation you were universally given about the Orthodox by non-Orthodox friends, and your own first-person experience with Orthodox Jews themselves. [I must add that the other black Jews with whom I am acquainted all share similar positive experiences.]
Either way, I've found that the easiest way to combat racism in the Orthodox community seems to be head-on. It generally is children - particularly teens - who will make comments, and when adults don't react, they think their comments have tacit approval. Speaking out strongly whenever hearing such comments has a strong effect, particularly to kids who have simply never heard much about other races (or religions) other than jokes or mocking. They simply don't know better.
Oh, and obviously welcoming black Jews into the community a little better would go a long way as well, though the ones I've met or seen in places such as Harrisburg and Cleveland seem to be pretty comfortable within those communities.