Are you sure sladeshot isn't working for you?
I tested your creature, and sladeshot worked just fine for me without edits.
In fact..
Flayer 1 gestures!
You fall unconscious!
Flayer 1 breathes a glob of slade!
The spinning slade strikes You in the head, bruising the muscle, jamming the skull through the brain and tearing the brain!
You have been shot and killed.
Meph has already addressed the CAN_DO_INTERACTION issue.
[CDI:INTERACTION:BLISTERS] and [CDI:INTERACTION:UNCONC] aren't needed.
As Alem said, you're missing the SEV tag for blisters and unconsciousness, and you haven't defined a body part for the blisters to affect.
SYN_NAME needs to be placed under [SYNDROME] to work.
These are working edits;
interaction_flayer
[OBJECT:INTERACTION]
[INTERACTION:UNCONC]
[I_SOURCE:CREATURE_ACTION]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_MANUAL_INPUT:target]
[I_EFFECT:ADD_SYNDROME]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_NAME:unconciousness]
[CE_UNCONSCIOUSNESS:SEV:10000:PROB:100:START:1:PEAK:30:END:1200]
[INTERACTION:BLISTERS]
[I_SOURCE:CREATURE_ACTION]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_MANUAL_INPUT:target]
[I_EFFECT:ADD_SYNDROME]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_NAME:evil blisters]
[CE_BLISTERS:SEV:10000:PROB:100:BP:BY_CATEGORY:BODY_UPPER:ALL:START:1:PEAK:30:END:1200]
[CASTE:DARK]
[CASTE_NAME:dark flayer:dark flayers:dark flayer]
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Cast Sladeshot]
[CDI:USAGE_HINT:ATTACK]
[CDI:MATERIAL:INORGANIC:SLADE:SOLID_GLOB]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:150]
[CAN_DO_INTERACTION:UNCONC]
[CDI:ADV_NAME:Cast Unconsciousness]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:20]
[CDI:VERB:gesture:gestures:NA]
[CDI:TARGET_VERB:fall unconscious:falls unconscious]
[CDI:WAIT_PERIOD:10]
[CAN_DO_INTERACTION:BLISTERS]
[CDI:ADV_NAME:Cast Blisters]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:20]
[CDI:VERB:gesture:gestures:NA]
[CDI:TARGET_VERB:break out into blisters:breaks out into blisters]
[CDI:WAIT_PERIOD:30]
I just used BODY_UPPER as an example for blisters, you can change that to any other body part you want, and add more if desired.
Also, it can take a while for the blistering to start.