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.
Edited to add: Still not fixed as of 2019/10/18 (although I did spell the notepaper as two words, note paper - I forgot and next time I should spell it as one word.).
And thanks for the lingot, whoever you are.
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.
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?
Yamir, I also find her pronunciation extremely hard to understand, but before you made your comment, teribleT had already provided the transliteration so that should’ve helped you know what was being said. In every language, all words are blended together and people only usually pause for emphasis or when a new sentence is starting.
Teachers in a language course will often speak artificially slowly, which is nice for comprehending new vocabulary but bad for understanding native speakers.
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.
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