Using an unholy combination of Pixipedia and Google Translate, I figured out that this embarassed girl is one of the playable characters in boyfriend_(kari). The pixiv tag for her is 青主人公 (or 青主 for short), which apparently means something like "blue main character." Judging by post #1841150, we don't have a tag for the playable characters from girlfriend_(kari) either...
These trousers have been drawn based on those jeans with the built-in bikini bottom. Occasionally see pictures of a Japanese model wearing them, floating around on image boards and the like.