Antihistamines, especially "sedating antihistamines" like diphenydramine (Benadryl) which can be found in many cold and allergy preparations, can caused blurred vision. They due this because the inhibit the signalling of the so called cholinergic nerves (they are "anticholinergic) which are used in helping the pupil adjust to objects at close distances. Therefore, if you are taking something containing one of these antihistamines, it is possible that you might have blurry vision. Other side effects might include dry mouth and drowsiness. The non sedating antihistamines like loratidine and cetirizine are much less likely to cause these side effects. Allergy eye drops (such as the "anti red eye" drops you can get over the counter) might also cause intermittent blurry vision.
As there are many other causes for blurry vision, I would suggest that you go get checked out by your eye doctor
or your doctors
">primary care doctor, especially if you stop the medications that you are taking for allergy and the symptoms continue. Additionally, if you are not able to take these allergy medicines due to side effects, you might need your doctor's help picking out a different medication that will be effective without causing side effects. Good luck with this!