Blitz Hinders Skill

Speed kills. Chess players are prone to error when they are under time pressure. In blitz, the time pressure begins with the first move. Games start in the normal fashion as players make moves they have made many times before. Some games may go twenty moves or more without either player being forced to confront an unfamiliar position. Some endgames, too, can be played very rapidly with little error.

In complex middlegames, however, blitz is groping in the dark. There is no time for analyzing the position. There is only time for reacting to concrete threats or for playing generic principled moves without concrete analysis of the nuances that are unique to the position on the board. Often there are familiar patterns in the position, of course. Strong players see more of these patterns and consequently make fewer errors.

Weak players reinforce the bad habits that make them weak. Improving players stagnate.

Blitz can help players improve. In one of the many chess forums discussions on the merits of blitz, a strong internet player offered a succinct statement on the principal benefits:

Blitz is good for getting a sample of what to expect against your opening repertoire. It also helps with exploring your intuitive tactical vision to see where you need work.
FirebrandX, "Does Blitz Actually Help Your Chess?" Chess.com

If you play enough blitz, you should encounter every likely response to your opening repertoire. That can be a useful addition to one's training regimen. To benefit, however, it would seem that a player needs more than just the experience of playing against every likely response. Playing the best moves in reply would be useful experience. To gain this experience, it is helpful to save blitz games into a database and review them. Check the lines against opening monographs and encyclopedias. Check these lines against a database. Check key moves with an engine from time to time. Play through master games in those lines.

Post-game analysis of blitz games helps to reinforce any lessons available from the opening. Such analysis seems even more critical for determining areas of one's intuition that need work. Here, too, a database can be a useful tool. During post-game analysis, if you see you failed to account for a bishop tucked away on a7, you can search your games to see if this problem is a recurring one.