What resources would you suggest to start learning visual design from for someone well established in UX and interaction design up to the black-and-white wireframes level?
Some resources you may find helpful:
50 Totally Free Lessons in Graphic Design Theory is a good place to start.
"While many of us can create something that looks good in Photoshop or attractive when spliced into CSS, but do we actually understand the design theory behind what we create? Theory is the missing link for many un-trained but otherwise talented designers. Here are 50 excellent graphic design theory lessons to help you understand the 'Whys', not just the 'Hows'."
A big part of the puzzle is just learning the tools. Almost everyone uses Adobe, but you have to choose your poison.
- Illustrator. Scalable/vector, best often if the work needs to be printed.
- Photoshop. Bitmaps. General purpose.
- Fireworks. Heavily oriented towards web work.
I am a photoshop guy and I am constantly boggled by Fireworks and Illustrator. Choose carefully and then do tutorials on the tool. Once you learn the tool, then it's an issue of art theories like composition, balance, color theory, alignment, yadda yadda. I love the book Universal Principles of Design to give an overview.
Online, SmashingMagazine.com is a wonderful resource for all things graphic design.