Biology

Would one example of that be white people moving to an area where black people live, and the resulting interbreeding making the population more vulnerable to skin cancer?

Does it though?

Lets break it down to simpler words. Gene flow is really just a fancy phrase for migration of a population. If a population migrates to another location, lets use Texas for example. Lets pretend Conner and his family, fresh off the boat from Ireland, get in a stage coach and move to Texas. They have a couple options;
A. isolate themselves and risk the environment
B. Mix with the locals and take up line dancing, margherita's and chili.

Irish tend to be sensitive to the sun. Sometimes it can cause melanoma's and then that leads to cancer and death. Not all the time though. What if Conner moves into a small Irish community in Texas and marries Siobhan and they have kids who marry within the community and have kids, etc, you have a population migration that has not adapted to it's environment so much as they have adapted to live IN the environment.

Could also be that Conner moves to Texas, meets Maria and sire 5 or 6 pups that don't get sunburn, but are super orthodox Catholic (or Protestant, that could also happen...) and therefore Conner's gene's have flowed all over Maria... ew.

Not all Irish people are ginger, and they don't all wear shamrocks and say "Bejabers" either.

You really love your casual xenophobia.

Are you sure?

It doesn't. Nothing prevents a population from adapting to its environment.

In order to prevent adaptation/evolution from occurring you would have to eliminate at least one of the following:
1. Reproduction
2. Variation
3. Selection

And since Gene Flow doesn't hinder any of those, it can't impede adaptation. If anything Gene Flow increases Variation, so it probably has the opposite effect.