I lived in the DC area for 25 years and don't claim to know all there is to know about it, BUT, especially for folks who have never been to the area, it is relevant to note that:
1. BOTH Montgomery County and neighboring Fairfax County are among the richest counties, per capita, in the country. This is accounted for in both instances by the high concentration of government bureaucrats who reside in both counties, black and white.Year in and year out the two counties compete to have their respective public high schools declared the best in the country, and they virtually take turns winning.
2. If you want to know the racial residential patterns that are relevant, they are the following. Generally, lower income blacks live in the Northeast section of the District of Columbia. Montgomery County is to the North and Northwest. Lower income blacks tend to move into the Maryland counties to the east of DC, Prince George's county for instance. But the geographical line blurs in the area directly north and continuing around toward PG county. It is impossible to draw meaningful inferences on the population of the two counties based upon race.
3. The biggest historical reason blacks have not moved into Virginia as frequently as to the east of DC is the invisible Mason-Dixon line which, politically at least, runs down the middle of the Potomac River. Remember, Richmond VA was the CAPITAL of the Confederacy.
It is an historical oddity, but remember that DC was supposed to have been created by taking a ten mile square from both Maryland and Virgina. FWIW, the Va part -- now known as Arlington County -- reverted back to Va sometime around the end of the 19th century. Look at a map. Draw a square around DC. It will overlap in the SW quadrant with Arlington county.
4. So what? History and politics explain more here than racial composition of the two counties. The blacks in western most portion of Montgomery County and Fairfax County have virtually the same demographics as their white brethren and sistren.
Income differences are more likely to be the engine for behavioral differences, and those differences don't track closely on a county by county basis because of the patterns I've described above.
The statistics presented don't make a powerful case for black v. white theories of crime because they don't show the income levels of the racial populations involved. Poor whites are just as likely to knock of 7-11's as poor whites, based on the data presented.
All the data shows is that areas with more POOR blacks have higher crime levels than areas with fewer POOR blacks. Go to Kentucky, Tennessee and my home state of W. Va. and you can prove the same thing with poor v. rich whites.