When relative path is used for indexPath in the LookupFactory, the solr cores DataDir is used as a prefix, but still will be created in the working directory of the process. Unfortunately both the BlendedInfixLookupFactory and it's parent AnalyzingInfixLookupFactory uses a store implementation that does not support HDFS. So in case the data dir is on hdfs then the "hdfs:/" prefix will be ignored and because of the missing leading forward slash it will be treated as a relative path. It seems hdfs wasn't really considered as an option when these Suggesters were implemented.
One possible solution is to use the SolrCores' own DirectoryFactory to lookup the data dir which dynamically will return the proper store in case of HDFS is used