a pop and a hiss

I was installing 6.0.3 to a hard drive when my stealth GS’s power supply gave a pop and a hiss and threw out a cloud of magic smoke. The IIgs kept going but I decided to shut down while investigating this. This is likely the original Astec power supply that came with it before the IIgs conversion. After opening the power supply up I found a component on the circuitboard right between the switch and power connector exuding black stuff. It’s marked 0,1uF on the top. It’s yellow, rectangular, about 1” long, ½” high and 3/8" thick. Anyone know what this is?
Tried the power supply on something I cared less about and it seems to be working though I didn't check voltages.