There is no difference. The two terms are used interchangeably. Glucose is the end product of carbohydrate metabolism. Glucose is a sugar. Hence, “blood sugar”.
In research circles, the term blood glucose is most commonly used and technically correct. Blood sugar tends to be used more in layperson articles and most media publications simply because it’s more understood.
The word “glycemic” in glycemic index refers to blood glucose. The glycemic index is the “blood glucose index” or “blood sugar index” and measures the impact foods have blood sugar.