As bananas ripen their starches turn to sugar so a less ripe banana shouldn't affect your blood sugar as much as a ripe banana. Ripe bananas do have a high glycemic index which means that it can spike blood sugar quickly. Your best bet is to test to see how eating a banana affects you. Choose bananas that are slightly green/yellow (without brown spots), and maybe just eat 1/2 if you notice that your blood sugar goes up too much after eating one.
One medium banana contains about 27g carbohydrates and 15g makes up one diabetic exchange so half would be a more appropriate portion for those watching their blood sugar.