We did the drive last Monday and found that there is a very large (and mostly empty) lot just before entering the town on the main road into Bled. Watch for the large blue "P" sign on the right. In fact, it was so large (and so empty) we thought that, surely, there's a "better" lot closer in. There really isn't -- glad we pulled in there!
It was 5 euro for 24 hours (actually, 25 hours, with an hour grace) -- pay at the machine and put the paper ticket in your window. We walked about a half mile into town and noted that along the way the closer-in parking was: not abundant (& this was at 8 am!), largely allocated to lodgings, and (when metered) typically costing 2 euro per hour (with no in-out like our lot offered).
Castle parking? We walked to the Castle and noted that the small lot was completely full -- glad we walked! Perhaps there is a larger lot on the back streets?? Don't know -- didn't come that way.
When we left Bled at 4 pm our lot was still mostly empty (only about 6 cars!!). We did note that there was parking at the far end of the lake (near the campground) but I think it was something like 10 euro for the day.