It is not clear to me why the front matter should contain a variable.
In most cases you do not need variables in the front matter. I think that this is also the case in your situation. I would move the front matter variable to the layout file. Thus, the layout file should look like this:
<meta property="og:image" content="myBlog/{{ page.url }}/{{ page.image }}" />
... and the front matter should look like this:
---
image: "b.png"
---
The SEO plugin can be easily exchanged by SEO without plugin, written by me. It is easy to adjust to your needs. Here is the code that it adds to the head:
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>{% if page.title %}{{ page.title }} | {% endif %}{{ site.title }}</title>
{% assign pagecontent_description = page.content | markdownify | replace: '.', '. ' | replace: '</h2>', ': ' | replace: '</h3>', ': ' | replace: '</h4>', ': ' | strip_html | strip_newlines | replace: ' ', ' ' | truncate: 160 %}
<meta name="description" content="{% if pagecontent_description.size > 10 %}{{ pagecontent_description }}{% else %}{{ site.description }}{% endif %}">
<link rel="shortcut icon" type="image/png" href="/img/icon-196x196.png">
<link rel="shortcut icon" sizes="196x196" href="/img/icon-196x196.png">
<link rel="apple-touch-icon" href="/img/icon-196x196.png">
<!-- Facebook and Twitter integration -->
<meta property="og:title" content="{{ page.title }}"/>
{% if page.image %}<meta property="og:image" content="{{ page.image }}"/>{% endif %}
<meta property="og:url" content="{{ site.url }}{{ page.url }}"/>
<meta property="og:type" content="article">
<meta property="og:image" content="{{ site.url }}{{ page.image }}"/>
<meta property="og:site_name" content="{{ site.title }}"/>
<meta property="og:description" content="{% if pagecontent_description.size > 10 %}{{ pagecontent_description }}{% else %}{{ site.description }}{% endif %}"/>
<meta name="twitter:card" content="summary">
<meta name="twitter:site" content="@{{ site.twitter_url }}">
<meta name="twitter:title" content="{{ page.title }}" />
{% if page.image %}<meta name="twitter:image" content="{{ site.url }}{{ page.image }}" />{% endif %}
<meta name="twitter:url" content="{{ site.url }}{{ page.url }}" />
<meta name="twitter:description" content="{% if pagecontent_description.size > 10 %}{{ pagecontent_description }}{% else %}{{ site.description }}{% endif %}" />
<link rel="canonical" href="{{ page.url | replace:'index.html','' | prepend: site.baseurl | prepend: site.url }}">
<link rel="alternate" type="application/rss+xml" title="{{ site.title }}" href="{{ "/feed.xml" | prepend: site.baseurl | prepend: site.url }}">
<link rel="sitemap" type="application/xml" title="Sitemap" href="{{ "/sitemap.xml" | prepend: site.baseurl | prepend: site.url }}" />
If you have any questions, please let me know.