The block heater actually has an O-ring on it. I would just replace it, since they're usually $30 or less. An old heater has the possibilty of popping out due to a corroded clamp, which has happened to me. I lost all my coolant without realizing it....drove it about 4 miles that way. Luckily no damage occured.
The worst thing about changing the heater is that you have to drain all the coolant. Clean the freeze plug hole really well with sandpaper. Put some grease on the new o-ring and heater and just gently tighten the clamp bolt down...don't overtighten.
Casey