I suppose you had a thorough eye exam from a qualified optometrist who is specially trained, especially important if you are taking Plaquenil.
I am wondering if what you are experiencing is more like a shimmery curtain effect than 'simple' blurring. Do you get headaches or feel nauseous or feel pain when the eyesight is blurry ?
If the optometrist couldn't see any problem it is more likely to be one of those temporary things that just happens. Make a note of when it happens to see if there is a pattern.
I can only suggest that you ask your G.P for an ophthalmogical appointment.
Sorry I can't be more help.