my image texture doesn't apply alpha I've been using Blender for a while now and recently when I import my logo which has a transparent background in image texture and plug it to my diffused colour, it doesn't overlay my logo on the diffused, rather, it adds my logo and change the whole object material to black.

I.e my alpha logo now appears to have a black background.
I had a similar issue.  I had to fix it with nodes.  Here's a screenshot showing how I did it.

After a log of experimenting, I found I had to add in a transparent BSDF into a mix shader with the Fac of the mix shader being the alpha color.

In the pic, the lower texture coordinate group did the trick for me.

If it doesn't work, let me know and I'll be glad to help.