I suggested this on Pandan's add on thread but here it is again.

For those, who don't know what it is, it is the legendary jar-based karate item in Team Fortress 2, that works a bit like a grenade, that explodes on impact. Jarated enemies take more damage for a short time and it reveals cloaked players.
Applying this to blockland as a weapon/item could be tricky so here is what I suggest to those, who want to make a simple version:
-players hit by the jarate will emit yellow water particles for a few seconds.
-a yellow flash on the screen (if possible)
-make the player really slow for a while (if possible)
-if slowing the player down isnt possible, it could just be made into a simple grenade weapon, that deals some damage.