It is just a very common situation. I can do this too. What you are doing is accomodating focus of your vision. The muscle in our eyes can contract or relax to change the shape of lens according to the oder from our brain. With the shape change of lens, its curvature is changed too, which lead to vision focus changed. If you know camera, you must know what I am explainning. Eyes are working the same way exactly with camera. With a camera, if you want to take a good and clear picture, you need to adjust the focal length. Taking pictures at different distance requires different focal length. If you are looking at something up close, yet you send orders from your brain to eye muscles, asking them to adjust the focus on to far distance, then your vision is blurred. Vise versa. No need to worry about it.
There is nothing wrong with your health or eyes it just means you have control over your eye the reason you can do this is you making your pupils bigger and small your eye automatically go smaller in light and bigger in dark it just means you making it bigger and you guys cant see In light while your pupils are big so there is nothing wrong with you, you just have more control over your eye muscles then most people and I am speaking from experience and if you wanna see for youself get a phone camera and put it close to your eye and blurr you vision while recording you'll see for yourself
From what it sounds like, you have nothing to worry about from a medical standpoint. As Debbie mentioned, the voluntary blurring of your vision is merely the result of your ability to contract or relax your ciliary muscles- the muscles that allow you to bend the lenses of your eyes to focus light onto your retinas. I doubt there is anything wrong with the health of your eyes. Voluntary ciliary muscle control has nothing to do with low blood pressure or diabetic complications. I wouldn't worry about it.
There may be the eyes inflammation for you. That is why you could make the blurry vision. There must be the invisible bacterium in the eyes. You could use some eye drops to release the symptom. At the same time, you could eat more food with vitamin C to moisture the eyes. After protecting your eyes carefully could you have the clear eye vision.
There must be something wrong with your eye health, Because we usually don't have blurry vision unless we are very tired or got some eye disease. Perhaps your symptom is resulted from low BP or diabetes ,etc. Here I suggest that you try to consult a doctor and take some treatment before it is too late for you.And please focus on building a healthy body.