Question about old movies re-released on Blu-Ray

[chipmunkee]

I apologize in advance if this question has been asked before. I did a brief search but the keywords I used seemed to be too broad.

I've always wondered about what is different about old movies that are re-released in HD format, specifically those released before Blu-Ray/HD-DVD were even announced. For example, take 'Blade Runner'. That movie was filmed over two decades ago but was re-released on Blu-Ray recently. I have not seen it on Blu-Ray but from what I hear it is much better than the VHS/DVD version. I'm just curious about how this can happen. Was the original movie filmed in HD format (if it even existed?) but compressed down for VHS/DVD? Or was the original movie filmed in another format that was still better than what VHS/DVD's could hold? Or did they go back and enhance the video/sound just for Blu-Ray? What does the term "Digitally Remastered" mean and does it apply to old movies on Blu-Ray? Basically I'm wondering how are movies that were filmed years before HD technology supposed to be "better" on Blu-Ray?

Sorry if it's all too much but I can't seem to find a resource that explains this.