Is there any way to manually turn off the "service engine soon" light on a 1999 blazer?

 

you can usually reset it if your disconnect your battery.

 

There is a reason the light is on. Go to your closest Advanced Auto or Autozone and have them read the codes. Once you have the codes that are stored, come back here and we'll help with the diagnosis.

 

Kyle is right. Have it checked at an Advance Auto Parts for free. They'll run the code and look through their manual to tell you where the issue is generatedfrom.

For future reference:
Disconnect your negative battery terminal for 15 minutes. This will reset the computer in your vehicle which should clear whatever code was thrown; however, if the light comes back on then there is definitely a problem.