It seems that these black lines appear randomly in the drawing. Is there anything I can do so that become gray again?.
I assume you'll need to edit your sheet format and fix them. They may be in the wrong Layer, or their properties may have been overridden with the Line Color function on the Line Format toolbar. After doing that, be sure to save your sheet format, and update any drawing templates that use this sheet format.
If that doesn't help, can you attach a Drawing with these black lines? (See How can I attach a file to a forum post?.)
Edit the sheet Select the Lines and add them to the correct Layer
if you don't use layers
there is this tool.
Retrieving data ...