I had two blank tiles at the beginning of the game. The other letters were not high scoring letters, but I was able to use all seven, for a total of 59 points. Would it be a better strategy to use the blank letters on two different turns, in order to maximize score?
People often consider a blank to be "worth" 25 points to a good player. That is to say, even though it has no points value in itself, it can improve your scoring possibilities by about 25 points through, for example, greatly increasing the chances of using all your tiles.
There's no single answer to your question and it will depend on the other tiles in your hand, but one way to approach the decision would be the following: if you could assemble a score of more than 34 and still keep a blank in reserve (i.e. the 59 that you could score, minus 25 for the retained blank), then it would be worth doing so. Otherwise, play your bonus word.
To add on @user2390246 's excellent answer, consider also the tiles you would be left with whether it will give you a likely score the next turn. If not, it is still not worth it to keep a blank for a score lower by your threshold (25 point in the answer earlier).
As example, say you have the tiles AAQUU?? (? are blank tiles), and you have the options to play sUbAQUA (lower caps are designated letters for blank tiles) for 85, or QAt for 65 keeping AUU? . I wouldn't save the blank because AUU? will unlikely net you high score next turn and there are not many chances of forming 7-letter bonus words with it either.