Using HTML in Blackboard - A Quick Tutorial
The following is based on Netscape Composer, an HTML editor. The principles are the same with any HTML editor.
Start Netscape, then start Composer.

Make a table by clicking the Table icon, or selecting 'Table' from the menu.

You can make and use complex tables if you wish, but for basic Blackboard postings a simple one-cell table works fine. Even if you use a complex table, start with a one-cell table on the outside, then put your other tables within the outside table.

Create your document. Use any of the features offered in Composer, but some may not work. Blackboard is not Internet Explorer, and it is not designed to do everything. In general, tables, font attributes, and numbered/bulleted lists work. Images work if you do it right (see below).

I changed the background color (see below).
I also added an image. Here's how. First go to the image you want to add. It must be on the Internet somewhere.

Right click the image, select 'Properties'.
Highlight the URL of the image, then copy the URL. Use cntl+c, or right click 'Copy'.

Going back to Netscape Composer, insert the Image.

Paste the URL in 'Image Location'. Either add some alternate text (anything), or click the 'Don't use alternate text' button, then click OK.

Here's what the Netscape Composer document looks like now (except for the green text I added as a graphic).

Now it's time to move the document to Blackboard. Click on the '<HTML> Source' tab.

'Select All', then 'Copy' the HTML code.

Go back to Blackboard, enter the Discussion Board forum of your choice, and select either 'Post a New Thread' or 'Reply' to an existing message. Paste your HTML code into the Message box, and click on the HTML button at the bottom. Before selecting 'Submit', be sure to change the 'Subject' line to briefly describe your topic.

Click 'Preview' or 'Submit' as you prefer. (Did you change the 'Subject' line?)
And this is the result.

It does take extra steps, but you'll like the results of your posting better than if it was plain text.