I have a 2000 Chevy blazer with the 4.3 in it only cranks.

Do the fuel pump test FIRST! .. DO NOT start looking for anything else until you know for SURE the fuel pump is turning on for 2 seconds when the key is turned on.. GM’s passkey III security system will not keep fuel pump from operating. IF the Blazer does indeed have the passkey III security system ((many did not)) the security light in dash would be flashing if system is activated. IS the security light flashing? Post these results for more help with the next step - if needed.