why are pictures dark and blurry indoors?

Pictures indoors and any lighting are dark and blurry

Chances are, it's because the flash on a typical point and shoot is underpowered. They're designed to capture largely portraits and small groups in low light situations. So, beyond about 6-10 feet, the flash dissipates to the point where you end up with dark photos. And the blurriness comes in by the camera choosing longer shutter speeds to compensate. Your best bet is to choose available light presets which will open up the camera wide open and boost the ISO speed. But that has it's own tradeoffs as high ISOs above 400 will introduce more noise into the image. Cameras with Image Stabilization combat this as well.

But most likely, underpowered flashes are still the most likely culprit here and would be so with just about any point and shoot.

I have a Canon SD-750. from time to time, when shooting indoor, the picture appear very dark.
I don't think it's a matter of a distance, because when shooting the same picture twice it appears great.
at the local lab, they were checking my camera reseting the camera, then shooting 200 pictures, claimed all of them were ok.
the next day, the problem appeared again.