One of the mods responded below that there is no specific meaning, it's just a practice sentence.
Blue strips may refer to the multiple choice test answer forms that many students have to buy. Or the blue books for college essays. Or just something similar. But for the record I'm just guessing I've only ever lived in America
Here is written: slips... That sounds strange. It is the fakse word, a wrong typo
Blue notes is a jazz word, but in the context of the lesson: "blue books" instead?
At least the english sentence is fabulous - what about תווים כחולים? Just blue notes.
I'm getting "They sell blue notes," which is something nobody would ever say in normal English. Why not "notebooks"?
It seems that מחברת and פנקס are more associated with 'notebook' and 'notepad'.
Trevorist's idea seems likely; otherwise, we do have 'sticky notes', 'post-it notes', and the like. It's not hard to see how the word form for 'note' could extend from the idea/conceptualization of written content, to the physical writing, to the sheet or slip of paper (with our without the content), etc. A teacher to students who are passing notes in class: "Hey, hand me that blue note!"
I'm sorry, even after reading the comments here, I have no idea what 'blue notes' refer to. If it's referring to 'notes' you write in your notebook, how can you 'sell' them and how can they be 'blue'? As an Australian, this is meaningless to me, unless it means 'notebooks'. Could an Israeli please inform me?
It doesn't have any specific meaning. It was created more to practice the word פתק. I see how this type of sentence is counterproductive, I will make all efforts to retire it in the next iteration.
Where I live (Greece), it is not mandatory to attend undergrad university classes, or at least certain categories of classes. Someone then can sit for the exam in the end of the semester without having attended some hours of teaching, or, in some cases, not even having seen a certain professor before. So, it is very common to look for people who have attended a day you missed, or a whole class even, and ask to copy their notes. I haven't heard of anyone selling those notes, but it doesn't seem to me completely impossible.
So, the phrase, they sell notes doesn't sound too strange to me. I can't provide a context for blue notes though, perhaps someone else has something to say about blue notes and we can combine them :-D
I think "blue notes" are copies of written notes. Up to quite recently a student would write her exam on papers with the papers behind it waxed with blue, so that she would make copies as she was writing: When finished she would give the originale and one copy for the school and one copy for herself. In past days, before the computer and the copy mashine, the same tecnique was used by authors and secretarys.
I just learned from another sentence in this lesson, from a native Hebrew-speaking Israeli (NaftaliFri1), that a peteq פתק is the note paper - and NOT what is written on it. Like post-it notes or small cubes of paper. English thinks of it as the content of the note, but Hebrew, going way back to when petaqim were made of parchment, thinks of it as the material written on. Next time you encounter him, you may want to confirm it with him.
Based on this, I move that we keep the sentence and translate it as "They sell blue notepaper" and that פתק have the note under it as notepaper.
I hope this helps.
Likewise based on another sentence in this set, I assumed this meant blue note pads or blue paper (like post-it notes coming in a little cube, and they're blue). So I tried, "They sell blue note papers," but DL marked it wrong. I am going to report this whole conversation to them.