Content writing is the second reason. Why?, because if you can design, but no contain is displayed, then it's a failure, I mean, the blogazine will not complete if you don't put a contain on it. For me, this is the hardest part, because sometimes I only have the layout and design, but I don't have anything to write. So this is the second, if you're good in writing, then no problem.
Layout is really important in blogazine. Without layout, a blogazine is not a blogazine. Layout is how you combine image, content and some element. This is hard if you're not experienced in blogazine, even for a professional bloagziner, making layout is still hard. This is the third reason, if you're having trouble with making layout, you can find inspiration from google, it's not about finding the layout that inspirated from a blogazine, you can just find a design such as poster design, web design, banner design etc, and then just make it with code not with image.
Choosing a matching color is not that easy. Color and content are things that connected, you have to find a color that match with the content. This is not always true, but yeah, if your color and content are matching, it's better. Also choose a color that matching with the layout, combining color is also hard. Me, I always use a low saturated color, because I like something vintage. That's the 4th reason.
Combine them all
Combine them all is the hardest. Because if you want to make a beautiful design, you have to make them all matching. This is the final stage, this is all, this is what's your blogazine post.