Blackfield applications are a minefield, reaking of smells and awash with technical debt. The codebase is a living hell.

Your first plan of attack is a plan. Your boss (be that you, your manager, your client or whoever) needs to understand what you are trying to achieve and in what time. Your team needs to know what the plan of attack will be and where.

Start with the greatest pain points, what are the biggest areas of technical debt, what takes the most time to work with\change and where are the areas with the higest number of defects. Work out what classes\functions are mud balls and where all the hard dependencies are. In working out the pain points you will begin to understand structure (or lack of) and where the fundmentals are. If know one in the team knows an area then profile it, understand what lengths the code is going to. 

When your done drawing up the list then work out what the common problems are, is the code hard tied to the database, file system or some other hard dependency. Is the code repeating it's self in structure\form over and over etc. From the list work out what are the areas with the biggest number of problems and make those your starting point.

Now you have a plan of what needs to change and where then you can work out how it fits into your development plan. Manage your plan, put it into a defect tracker, work item tracker or use notepad or excel etc. Mark off the items on your plan as and when you have attacked them, if you find more items then get them on your plan, keep the movement going and slowly the codebase will become better and better.