Honestly people won't give a sh*t what you sub as WHM, really, nor will most of them even know the inner workings of the job and why /SCH is overall a better sub in the first place. People look at WHM and they think Cures, status heals, Hastes, Teleports, and Raise 3. As far as they are concerned, you have all of that no matter what you sub and for the most part, they all work equally as well regardless of your sub.
For WHM the sub just supplements the job and isn't really essential to making it work. The same often cannot be said for say, tank roles. No matter what main job you're trying to tank as, the sub is usually essential to making it work out well if the thing you're trying to tank is even remotely challenging.
Bottom line, is career /BLM use "acceptable"? Sure, it must be, considering I still see many career WHM/BLMs make it to and thrive at 75 to this day. However, you will have a harder time with the job than someone with /SCH and your MP stamina will be poorer, particularly in scenarios where a RDM is not present (and given the current trend of whoamg TP burn, you are going to be very unlikely to find parties that will invite both a WHM and RDM after around ToAU levels). Whether or not it's "OK" to maintain usage of /BLM is something you should be answering, not leaving it to other people. If you feel like you would want better MP conservation as well as the option of a mini-Refresh (Sublimation) when you cannot get the real thing from a RDM, then you level /SCH. If not, then don't. As far as everyone else goes, as long as they're getting their Cures, status heals, Hastes, Teleports, and Raise 3s, they couldn't care less what you do.