USPS does collect mail from the blue boxes daily, except for Sunday.
USPS collects mail from the blue boxes once a day on days Monday through Saturday, on the posted times which can vary.
The actual time of day that USPS collects the mail from the blue boxes is usually at the end of the close of business for the post office.
For example, at my post office, the post office closes for the day at 2:45 PM, which is also when the mail is collected from the blue mailbox at the post office.
So for the blue USPS mailbox at my post office, the mail is collected by 2:45 PM which is the last collection time.
Some USPS blue mailboxes are collected at 5:00 PM or 4:45 PM depending on the location.
USPS blue mailboxes that are spread out through a town are often collected at the end of the of the day although during busy times such as holidays when there's a lot of mail, the USPS may collect the mail a few times a day.
The time that the USPS collects the mail from those USPS blue mailboxes will be stated on the mailbox itself for that individual mailbox.
As long as you get the mail put into the blue mailbox for USPS before the last pick up time, then the mail will be collected and go out that day.
However if you put the mail in the blue USPS mailbox after the collection times, then the mail will be collected the next business day and go out then.
The mail in the USPS blue mailboxes is only collected on days of Monday through Saturday and not on Sunday.
If you put mail in the blue USPS mailbox on Sunday it won't be picked up or collected until Monday.