Is it assur to make a bracha on foods whose ingredients are kosher but cooked in non-kosher utensils? What is the source?
The Shulchan Aruch writes (196:1):
אכל דבר איסור אף על פי שאינו אסור אלא מדרבנן אין מזמנין עליו ואין מברכין עליו לא בתחלה ולא בסוף׃
If one ate something prohibited, even though it's only prohibited Rabinically, we do not form a zimum on it, and we do not say a beracha before or after eating it.
The key thing to note is that it is not necessarily a food's "kosher" status that governs whether a bracha is said. Rather, the key is whether it is permitted or forbidden to be consumed.
So in the case of food cooked in non-kosher utensils: it would appear (according to my understanding) that if the circumstances are such that it's forbidden to be eaten, one cannot make a bracha on it.
I should note that I have seen in the Aruch Hashulchan that there are certain exceptions to this rule when it comes to "lighter" Rabbinical prohibitions. An example he gives is בישול עכו״ם. I am not quite sure if this would have any bearing on the utensil case...