Markerattrs=(symbol=circle) or markerattrs=(color=black) does work as expected. You just cannot assign a list of symbols or colors in this way.
ODS Graphics automatically applies the settings from the style to the group colors, symbols and line patterns. By doing this, all graphs in your report will have consistent attributes for the same group value. These colors, marker symbols etc have been carefully chosen by GUI experts to provide maximum clarity and discriminability between different groups when using the default styles.
Furthermore, there are many styles shipped by SAS for you to use. The whole idea is to create aesthetic and effective graphs out of the box without users having to spend time finding good combination of colors. Of course, if you need, you can design your own style too.
If you are not using a group variable, you can change the setting of the plot by using the markerattrs=(symbol=circle, color=black) or whatever you need. If you have a graph made up of multiple scatter or series plots, one per column, you can control the color and other attributes of each plot by using this feature.
In future release, there may be a way to set group colors, markers, etc in the syntax itself.