Why does elevated levels of K+ in blood bring a neuron membrane potential closer to threshold?

You are sort of right but i don't think you are in the right state of mind. I will make it as simple as I can.

This is what happens in a typical excitable cell action potential. The cell gets stimulated and sodium channels open creating the depolarization (spike). Sodium rushes into the cell (lots of sodium normally outside of cell) creating a positive internal environment. Sodium channels then close to stop sodium influx. We are now at the peak of the spike / most depolarized / most positive part of action potential.

Here is where potassium comes in. To repolarize the cell back to its resting membrane potential potassium channels open and potassium (normally high inside of the cell) leaves. If there are elevated levels of potassium in the blood... less potassium is going to leave the cell. Thus the inside of the cell will remain more positive then it should be. This heightened positive state is closer to the threshold.