Ink, and to a lesser extent, varnish traceability is very, very difficult and every printer, large and small struggles. The auditor would be very mean to challenge your ink traceability systems on a first audit. There are however, mean auditors about.
I haven't got an easy solution, but try to think of the ink department as a separate factory which makes finished products from a recipe and sells the print department finished products. if you were starting from scratch you would set up batch control systems, if only to control waste efficiencies.
You don't say how many blends each day that you make. Using a simple excel spreadsheet is a starting point if there are only about 10 or less. Getting the ink blenders to record the batch numbers of course is a different matter.
One point I want to make about Peter Snopko's post is that tracing back from blend to primary inks is not the problem. It is the forwards trace from primary inks to blend and then to finish product which is more important and also much more complex. Although not a requirement of the BRC/IoP the ability to perform a mass balance reconciliation is what you should be aiming for.