Can I block a field within a Matrix?

I am trying to block the 18MG on some of the rows so people cannot enter information.  Can we have it blacked out?  or an "N/A" option?

Unfortunately no. You cannot apply condition rules that can be specific only to a cell in the Matrix table.

Usually the workaround used is by using a separate Textbox field (outside the matrix table) or use another Matrix table with only single column (just for the purpose of having a separate textboxes)

OR

You may want to try using Spreadsheet Widget as an alternative to Matrix table.

link: http://widgets.jotform.com/widget/spreadsheet

demo form: http://form.jotformpro.com/form/51730795061960?

The spreadsheet widget will allow you to have control on the cell contents. You can assign a default value to the specific cell address.

Hope this help. Let us know if you need further assistance.