Quote Originally Posted by Bunny
...specifically because he did not know exactly where it came from. He could have deduced (rather easily, being one of the lead detectives in the department) that the blood was Liddy's based solely on that fact.
With absolute certainty (or at least enough certainty to publicize that fact to Dexter)? There's quite a difference between a reasonable conclusion (which this was, certainly) and being without any doubt whatsoever. Quinn's apparent reaction to the results was apparently not "oh god, I was worried that was Liddy's" but "that was definitely Liddy's blood and Dexter botched it to protect me!"

What puzzled me was how easily it was noticed by LaGuerta, as it was a rather small speck of blood and she just randomly happened to look down at it.
I thought that was the most reasonable part. LaGuerta just stared down for a moment -- not an uncommon reaction -- and she's a cop. Cops look for and and see blood all the time. It seems reasonable to me that her eyes glanced over the stain, she unconsciously focused on it as an irregularity in the shoe, and then a couple of seconds later realized it could be blood. She already had plenty of reason to be suspicious of Quinn anyway, what with the calls from Liddy and not handing over the messages.

EDIT: Now that I think about it, the blood not being Liddy's would hardly remove Quinn as a suspect. I could understand that it means they don't have enough to keep him locked up anymore, but he still didn't provide any reason for all the calls from Liddy right before the murder, did not hand over any messages (because he deleted them), and it was still his name signing out for the surveillance equipment. I wonder if he'll still be suspected in season 6, though this plot will probably just be swept away.