The breather canister is not a sealed unit, so by definition it leaks air. Ask why there is oil leaking... Does it leak at idle, probably not, then it makes little sense to check at the fill cap at the dock at idle... if you agree... then to trace the problem you need to SEE what is going on with the valve cover (s) removed. The link I posted explains what to look for. If you get leakage during rough sea-state vs calm water conditions while WOT under load, then that adds to oil pooling evidence.
In the end I added Racors and upgraded the valve cover to the P style and piped the gases into the filter. I fixed my problem, so my Racors are clean, thats another discussion, so suggest if you go this route, you dump the expelled oil into a bottle and see whats going on till you find the root cause of this leak. Not sure I would keep calling it blow by without confirming compression failure. Check cheap and simple first, especially if this just happened. Also ask yourself if something changed during maintenance or if the engine got over heated, etc.