check www.bahn.de - the German railways web site which has train and often buses acting as trains as in this case - a good way to access it is to go to the home page of www.budgeteuropetravel.com and click on the link "Best online all-European railway timetable (or some such wording) and this brings you to the English schedule page where you just put in Venice (Santa Lucia, main station on a canal) and Bled and date and you should get all the connections. I suggest this home page link because they also give you valuable info on fully using the Wunderbar bahn.de site that may not be apparent at first glance. You may also be able to go via Trieste and Lljubjana.

You have Slovene bus company DRD. Goes from Venice-Mestre to Ljubljana. In Ljubljana change to local bus, every hour to Bled.

the bus Venice to Villach and then train to Leske Bled (station to put in on bahn.de) takes all told about 5.5-6 hours.

I have good experience with DRD, they leave Venezia Mestre station at 11:30, and arrive to Ljubljana at around 2 pm, depending on traffic. It is a van for 9 people. From Ljubljana, take a train to Bled:
http://www.slo-zeleznice.si/en/passengers/slovenia/timetable/timetable-with-prices?entrystation=42300&via=-1&exitstation=44716&date=15.6.2012&submit_ttable.x=34&submit_ttable.y=13

Do the Soca valley, much more scenic!
Train to Gorizia, over to Nova Gorica, train to Bled Jezero via the Soca Valley and Mountain Pass.
Excellent.