I have a Canon 60D + 50mm F1.8 lens
I shoot today. Damn!! Over exposed!!...I wanted the blurred background look
The aperture was pushed up to a higher value and no longer changeable...What am I doing wrong?
This is a very common problem, especially for new DSLR videographers. It can be especially confusing if you're used to a camcorder. On a consumer camcorder the depth of field is deep, so you don't lose much by stopping down, some have built-in ND filters, and more adjustable video gain.
A flexible solution for DLSRs is using a variable ND filter. On this variable ND shootout, the Tiffen was well rated. I use it on my 5D3 and it's very good: http://www.learningdslrvideo.com/variable-nd-filter-shootout/
The other solution is stopping down but using a longer focal length. I sometimes shoot interviews at 280mm @ f/4 on my 5D3 which is equal to 175mm on your 60D. If you had a 70-200 f/4, at 175mm you could stop down to f/8 at a 20 ft camera-to-subject distance and have about the same depth of field as 50mm @ f/1.8 at 10 ft: http://www.dofmaster.com/dofjs.html
F/8 is 4 1/3 stops darker than f/1.8 so that might be enough to maintain your shutter speed -- on a cloudy day, but not in bright sun.
As a last resort you can increase shutter speed and sometimes on an interview or static subject you can get away with it, but it's risky. It can cause strobing, especially if anything is moving -- even out-of-focus tree leaves in the background.
The problem is a sun-lit subject is incredibly bright. You don't notice this because your eyes are so adjustable. They can handle starlight at 0.0001 lux to a sun-lit beach at 100,000 lux -- a billion-to-one range. Without aids like a ND filter, a video camera cannot handle this.
At a fixed shutter speed and ISO 100, a sun-lit subject will need either 1/8000th sec or f/22 or some combination to balance the exposure. If you want f/1.8, your 60D might over-expose even at 1/8000th, it's maximum shutter speed: http://www.calculator.org/calculate-online/photography/exposure.aspx
I'd suggest something like a 70-200 f/4 lens, coupled with a variable ND filter. That gives you (a) More flexibility at subject-to-camera distance, (b) Ability to stop down and maintain shallow DOF, (c) Variable ND to dial down light and maintain a wider aperture.