I haven't had any major issue to style the OOTB "Add Comments" WebPart, it's almost like styling any other built-in NewForm/EditForm WebPart.
As for hiding a field: you could use PowerShell to set the field to hidden completely or hide it only on the NewForm/EditForm/DispForm pages. Check the SharePoint object model for the SPField properties
ShowInDispForm for more info.
UPDATE 2: To style text of the OOTB "Add Comments" WebPart on a Blog post page you could use Firefox with Firebug or Internet Explorer with the IE Developer tools (just hit F12) to locate the HTML element you want to apply your custom style. No SharePoint Designer (SPD) required at all.
UPDATE 3: Don't get me wrong on this. SharePoint Designer (SPD) is a great tool for a lot of tasks, but you need to know where its limitations are. To me SPD has been a great help of understanding SharePoint better and I occasionally use it for some light tasks but I tend to stay away from it for many things like XSL, CSS or masterpage editing because the SPD code editor is the worst I've ever seen (and almost remained ugly and unchanged since FrontPage 2003 about 10 years ago) and it has been crashing (and loosing unsaved work) more than once.
PS: I personally tend to always stick to the OOTB "Add Comments" WebPart because customizing Blog stuff can easily break internal things as you already discovered yourself.