Like you, I think some people have a really strong gift in this area, but I don't think that means people like me can't get there. :-) I love your way of doing the mock-up. I've done it unintentionally by making a block that doesn't work and then ending up having to rip it apart!
In terms of colour, I will often take my inspiration from fabric because I figure the designers must know something about it and if I like it in a fabric I'll probably like it in a quilt. There are websites like canva.com which can help, too, although I prefer the fabric method.
I don't generally buy fabric for a project - I'm on a rather limited budget - so I will take the fabric that I have that I think will work and pile it on the table and sort it into groups and walk away and think about it and then approach the table to see if there is anything jarring. Using the black and white filter on my mobile phone also helps when I need to figure out contrast.
I like your block, btw :-) Blue and yellow is a good combo in my mind. :-)