I'm scared Could a Volcano Eruption be so Bad that it Blocked out the Sun?

Well, yes there are volcanoes that if they erupt could cause similar effects. However I wouldn't worry too much about it, the year without summer was caused by a variety of combining factors capped off by the Mount Tambora eruption. Now you have to consider that the 1815 eruption of Tambora was huge, much bigger than Mount St Helens (for comparison). There's not really any volcano likely to erupt soon that will cause those kinds of effects.