Caffeine DOES NOT by itself raise blood sugar levels. However, it does reduce glucose tolerance; i.e. if you consume carbs with caffeine, you will have a higher blood sugar response. This was demonstrated in "Effects of caffeine on glucose tolerance: a placebo-controlled study" (1998) and "Caffeine-induced impairment of glucose tolerance is abolished by beta-adrenergic receptor blockade in humans" (2002). If you are on a ketogenic diet, and consuming zero carbs (or near-zero), caffeine will have no impact on your blood sugar levels.