The short version : How can I print an element (e.g. box) that has a transparency set at 50% without the output appearing dithered?

The long version: I'm building a training schedule so I've used the calendar to lay out the next couple of months. Each course has a colored block that's either 1/4, 1/2, full or 1 1/2 day long. I move the blocks around in order to come up with an ideal schedule. I've se tthe transparency for each block to 50% so that if courses overlap, I can clearly see the fact. Each block has the course number in 8pt typeface and the number of participants in 6pt (subscript). On the screen they can be read with no problem, but when I print, the 50% transparency comes out dithered making the small text unreadable. I've tried printing color as black but it's still dithered. I'd hate to have to change the transparency, is there a way to force Visio to either print the color or not print the color (and therefore no dithering)?