I have a 2000 Blazer 2 wheel drive, 4.3 manual transmission, and it has been idling strange lately, most of the time it idles just fine but then other times it idles at about 1500 rpm,

Does anyone have any suggestions ?

 

Try the throttle position sensor.

 

I would suggest cleaning the throttle body and the idle air control valve...
Make sure your air cleaner is clean and properly attached...
Check vacuum hoses for any cracks or tears that might be causing a vacuum leak...