Blood pH decreases
For this problem you need to remember what pH is. The higher the molarity of hydrogen ions, the lower the pH, or the more acid the solution is. The lower the molarity of hydrogen ions, the higher the pH, or the more basic the solution is. Now consider the question. You're having fewer hydrogen ions being secreted into the filtrate. That means that the substance being filtered has it's hydrogen ion concentration increasing. And the substance being filtered is blood. And since the hydrogen ion concentration is increasing, the pH is decreasing. So the answer is "Blood pH decreases".