I think the tingly feelings in the muscles could be from over oxygenation--meaning you may be breathing too fast. Try slowing it down some...maybe even count with a pause of 2 count or so in between inhalation and exhalation.

There are some types of meditation that allow for everything to just flow through your thoughts...allow them but don't hold to them, let them go. I daresay I've never seen anything dark or mystical myself.

I prefer to focus upon something to meditate, a purpose driven meditation you might say.