This question is essentially impossible to answer in any objective way. It would be prohibitively difficult, and perhaps impossible, to determine if black politicians are targeted for corruption more often than white politicians. Let us see why this is so.
To answer this question, we cannot simply count how many black politicians have been targeted for corruption (which would be hard enough given how many governments there are at various levels in our country) and then compare that to the number of white politicians targeted. We cannot even use look at what percentage of politicians of each race are targeted. This is because it is possible that black politicians are more likely to actually be corrupt than white ones. If more black politicians are actually corrupt, then it would make sense to have more black politicians being investigated.
This means that we will have to compare the rate of corruption of black politicians to that of white politicians. In other words, we would have to look at how many politicians of each race are actually guilty of corruption. We would then compare that percentage to the percentage of them that are targeted for corruption. So, for example, let us imagine that we found that 25% of black politicians and 20% of white politicians are corrupt. We would then expect that slightly more black politicians would be targeted. If we found that 50% of black politicians were investigated while only 25% of whites were, we could say that blacks are more commonly targeted. The problem, of course, is that we cannot ever know how many politicians are actually corrupt. Therefore, we cannot answer this question objectively.
It is possible that we could look at the ratio of investigations to findings of guilt for both races. If blacks are being targeted more than whites (without cause), we would expect to see a higher ratio of investigations to findings of guilt because more blacks would be investigated without cause. Even this, however, is essentially impossible because of the difficulty of finding out how many politicians across the entire country have been under investigation.
All that said, there has been at least one attempt to answer this question. In this link, we have an interview with a professor (himself African American) who wrote a book on this subject. He claims to have found that there is no conspiracy to target African American politicians for corruption. I do not know what his methodology was, but he concluded that there was no reason to think black politicians were being targeted because of their race. We must note, however, that his research covered a period that ended almost 20 years ago.
I do not believe that we can answer this question in any objective way. Most answers will be heavily influenced by the political viewpoints of the people from whom they come.