Those questions about Blazegraph perf tuning were asked on the wikidata mailing list by Thad Guidry. I'm not sure how much of that we've already done:
Is the Write Retention Queue adequate?
Is the branching factor for the lexicon indices too large, resulting in a non-linear slowdown in the write rate over tim?
Did you look into Small Slot Optimization?
Are the Write Cache Buffers adequate?
Is there a lot of Heap pressure?
Is the MemoryManager have the maximum amount of RAM it can handle? 4TB?
Is the RWStore handling the recycling well?
Is the SAIL Buffer Capacity adequate?
Are you not using exact range counts where you could be using fast range counts?
Start at the Hardware side first however.
Is the disk activity for writes really low...and CPU is very high? You have identified a bottleneck in that case, discover WHY that would be the case looking into any of the above.
and a 100+ other things that should be looked at that all affect WRITE performance during UPDATES.