There is something "fishy" here. The proportions to neutralize a gray is not putting less yellow.
For example. A Swop profile transforming a pure RGB black to CMYK gives this: 75 68 67 90. Let us forget the K. The magenta and yellow almost the same proportion (redish) and some more Cyan to neutralize that redish tone.
So yeap. There is a chance thoose values give a bluish tone. A more "logic" value could be something like 35 30 30 100. But do not use thoose values either.
The pre digital era rich black
There is a tradition dating the pre digital era, where the "Rich black" was using 100% k (that has logic) and becouse this ink is transparent, they needed aditional coverage. This was trial and error, or a "Just enough" aproach.
So a 30% cyan was offen used, but gave a cool black, so some magenta was recomended in some other cases. But this 30% was only an "arbitrary" number. If they only used 30% that was not too much cyan and it would not cool as much the black.
In this digital era
You need to use a Profile.
Either the printer (or more likley the manufacturer of the equipment) should have a specific color profile for the combination of inks and paper (or vynil or whatever).
This profile defines how the specific inks behave, what is the maximum amount of ink among other things.
Some profiles now uses a 300% ink coverage for "real" black, some goes for 320% or more. This numbers they give are just 180%!.
What you need to do
1) Ask for the profile they are using.
2) If they do not have a particular one, ask which generic profile you should use. Probably SWOP2, Fogra, Gracol.
3) Make a pure RGB black and convert it to CMYK. And use thoose values for your black. Again, for example doing this on Photoshop with a SWOP2 profile you have 75+68+67+90.
What if you want even further control?
1) Prepare some charts of the colors you pretend to use.
2) Ask for print sample of thoose charts.
3) Make adjustments acordingly.
A little warning
Do not go beyond the recomended ammount of ink.
This is becouse a lot of ink can make a mess on the printer heads, or could not dry properly, or the printed black could fall in pices.