Hi Adiyah,
Yes you can add alt tag in blog images writing your custom transformation.
Example
ASCX transformation code
<div class="blog-panel">
<div class="blog-content">
<div class="image">
<a href="<%# GetDocumentUrl() %>" style="display:block;">
<img src="<%# Yourblogimage %>" alt="<%# EvalText("yourblogtitle", true) %>" />
</a>
</div>
<a href="<%# GetDocumentUrl() %>">
<span class="ProductTitle textContent">
<%# EvalText("SKUName", true) %>
</span>
</a>
</div>
</div>
Thanks
Happy to help you