What does "滿腦門子的黑線" mean?
It's not popping veins, but cold sweat for frustration or helplessness, usually as a response to something completely out of the line or out of the context.

Wikipedia:


  Parallel vertical lines with dark shading over the head or under the eye may represent mortification, fatigue, or horror. If the lines are wavy, it may represent disgust. A far cuter way to represent frustration/mortification is (mainly for female/ young female characters) they tend to puff out their cheeks while their line is delivered in a gruff voice, an elongated 3 showing puffed lips to show that puffed look more.




Popping vein is visualized and then character-ized as '头上一个大大的井字'.