How do you keep the foreground and background from blending together?

[Nono]

The key is value, also contrast, not just color. Also as other people have said, different line weights help too.

But usually you should try to separate the composition of an image in layers of different values. A common example is when you see a picture with mountains far away in the distance, you see less detail in them and they look kind of washed away compared to the things in the foreground.
Here's a good video on the subject of values:

[Darth_Biomech]

The way a friend of mine explained it, the background should be separated from the characters by the contrast and the value of colors. Easiest way to check that is to put the whole picture in black and white and see if the characters pop up against the bg - usually they need to be noticeably darker than the background (Or brighter, if it's a dark scene).

Also please don't overuse blur. It's handy to simulate depth of field, or when you want a generic background with no details, but in every other instance it makes me want to check if my glasses are clean.