Although Distributed Host Table (DHT) and bitcoin blockchain are both distributed networks, both of them serve different purposes. While DHT divides data in a network by providing an efficient structure in terms of lookup time and storage footprint, bitcoin blockchain stands up for a tamper-proof distributed data network.
I will tell you how.
In DHT, keys are distributed across several nodes in the network with a deterministic algorithm. And each node is assigned with an identifier and this identifier can be used to find other nodes in the network.
Suppose there are 4 nodes in the DHT that have identifiers as a1b1, a2b2, a3b3, and a4b4. Node a1b1 will be storing data of another node that has an identifier a5b5.
Now assume that you ask the node a2b2 for data in a6b6. Although a2b2 does not have the required data, it connects with its nearest identifier and returns the required data.
This is how lookup time is improved, as addresses of each node are not stored in the DHT. Algorithms are used to find the required data in O(log(N)) jumps. This makes DHTs scalable because data is distributed uniformly amongst all nodes in the network.
In the case of blockchain, all nodes contain the complete data that pertains to the network. The process of mining is conducted to validate new entries to the network through consensus. Therefore, data stays in a distributed ledger but is in fact secure and tamper-proof.