boarders
It usually stands for rivers or borders, depends which map you're looking at.
A national boundry is a thick black line
The national boundaries
It is common for a map "key" or legend to gauge the size of cities on a map. For example, a city with a population of over 200,000 may be marked by a black dot surrounded by a black bordered circle line. White is the shade between the black dot and the black line of the circle.
Depending on the type of map you are referring to, the solid black line could indicate a national boundary. If it is a map of states, it could be a state boundary.
Contour lines on a map are typically brown or black in color. They indicate elevation changes on the map.
A stream,river or another body of water.
The Key, or Legend, or a map gives a list of the symbols used on the map and what they represent. There is a fairly standard set of symbols used for different types of map so that anyone looking at the map can understand what it is being shown. For example, on a weather map a cold front is shown as a black line with semi-circular bumps (in blue if coloured), a warm front is shown as a black line with triangles (in red if coloured) and an occluded front is shown as a black line with alternate semi-circular bumps and triangles.
A map contains many distinguishable symbols that are universal. A circled star on a map stands for a state capital.
The lines on a map for pressure stands for Isobars which they are lines on a map around areas with the same pressure readings to indicate highs and lows.
Depending on the type of map you are referring to, the solid black line could indicate a national boundary. If it is a map of states, it could be a state boundary.
The Black-Blue-Black design is usually seen on a policeman's personal car or family members car. It stands for the "thin blue line".